Zone 2 feeds can only be analog; there is no D/A conversion to feed zone 2. There are a few potential if somewhat clumsy work-arounds for this, but you will need to spend a little dough to make it work.
[ADDED: The simplest thing is, of course, to take the analog signal from your computer via 1/8" to RCA to the CD, TAPE IN, or AUX inputs. However, the sound quality that you will have depends on many factors - certainly the quality of the sound card in the PC, and its noise floor, etc. If you want to maintain the fidelity of the digital signal on the USB feed, then there are some options, but again, these are not the most 'elegant' solutions, but should work. See below]
Here are a few options to achieve what you want:
One method would be to take the front channel RCA outputs from your 990 and connect them to the amplifier that is in zone 2 (this is especially easy if you are using the balanced outputs of the 990, because that would mean that the RCA's are presently unused). The trouble is, you have to select the USB input (for the front room) so you would lose the dedicated zone 2 output, that is, it will no longer be independent - just a duplicate of what's going to the main system. Also, the volume on the 990 will control the volume of zone 2.
Another option would be to find outboard USB D/A that is capable of providing an analog output as well as a S/PDIF or a TOSLink output; I honestly don't know if these exist (it's worth checking the Outlaw site to see if they make something like this), but if they do, this approach should work. That is, you connect the computer to a USB interface that provides both digital and analog outputs. Then you could take the analog output of that converter, and feed it to one of the 990's analog inputs (AUX, CD, TAPE), and then assign THAT to zone 2. Then, you would connect the digital output from the stand-alone D/A to one of the 990's digital inputs; this would allow you to have two forms of the same signal (analog and digital) and thus, you could select the digital signal for your main room (or the analog signal) and of course, 'map' the analog signal to zone 2. Worst case, if you used an external D/A and could only take analog, at least you would have the ability to listen to that in your main system as well as send it to the zone 2 outputs. However...if such a device could in fcat provide a simultaneous digital output, this would allow you to kill two birds with one stone as you would have both types of signal from the box.
UPDATE: So, I looked around on the web, and while I have no idea what this costs, it will do that of which I spoke (above). That is, this mates to your USB port on the computer, sends the data wirelessy to a small (abouit the size of a few packs of cigarettes) receiver, and generates an analog output as well as a S/PDIF output. So, you could take the analog to the CD, AUX, or TAPE IN on the 990 (and assign THAT to zone 2), and then take the S/PDIF to one of the 990's S/PDIF RCA-type inputs (I have no idea if you are using both of the S/PDIF inputs on your 990... However, if you have used both of them already, you can find simple low-cost format converters (about $30) that will take the S/PDIF format and convert it to TOSLink.) So, provided that you have at least ONE open S/PDIF this alone would do it, or if both are already used, then as long as you have a spare TOSLink input, you could easily convert this device's S/PDIF to TOSLink, and you would be all set. Here is the link: http://nadelectronics.com/products/digital-music/DAC-1-Wireless-USB-Digital-to-Analogue-Converter
END UPDATE.Still another option would be to take the optical or s/pdif output from your computer (if it has one) to an outboad D/A converter. Many outboard D/A converters provide both TOSLink and S/PDIF outputs as well as an analog version.
So, suppose your PC has an S/PDIF output and the stand alone D/A box had both TOSLink and S/PDIF capability...in this case, you would take the S/PDIF feed from the PC into the stand-alone D/A, and chances are that if it also has a TOSLink connection, the digital signal presented to its S/PDIG would be available at its TOSLink output. Them from there, you would take the TOSLink to the 990, as well as the analog outputs from the D/A to the CD, TAPE IN, or AUX inputs on the 990, and map that to zone 2. Conversely, if your PC has a TOSLink output you could feed that to the external D/A, and (again) provided that such a box could provide a S/PDIF signal, you could take that to one of the 990's digital inputs, and (as before) take the analog signals from the outboard D/A into one of the 990's analog inputs.
I know that many D/A converters do this sort of thing (one digital format in and a complementary digital format 'passed through' to something like a TOSLink or AES/EBU in addition to the analog outputs). For example, it is possible to do something like this on Midiman's "Flying Cow", however, the digital interfaces are S/PDIF and AES/EBU, so if your PC had S/PDIF you would indeed be able to get the analog version as well as the digital version, but you would have to find an AES/EBU to S/PDIF converter (or an AES/EBU to TOSLink converter) and take that to the 990. Again, clumsy...but functional.
One other option - and I have NO idea if this will work - would be to try and take the TAPE OUT signals from the 990 and feed that to zone 2. However, this would (if it works) be a fixed-level signal (you could not adjust its volume on the 990), so you would have to control the volume downstream of it. However...I DO NOT know if the 990 provides an analog copy of the digital input on its TAPE OUT connections, or if the TAPE OUT only provides a signal that is fed to the AUX or CD RCA's.
I don't know if this helps...or if it muddies the waters. Anyway...I hope it helps, and if you have further questions, feel free to email me.
Mark