Usually on motherboards that "support" 5.1 but don't have that many audio outputs, they rely on an (HD) Audio connector that you hook up to the front panels (or its equivalent) audio outputs. And then via software, you can configure this normally different input/output on the front to be the other missing two channels. The other approach is whilst there is offcially only "two audio outputs" there are actually 3 sockets (2 output, 1 input) at that back of the motherboard you can hook up and they are all configurable to become all outputs (for left/right, surround left/right and voice/bass) or even inputs. However, not all motherboards do this, so you are correct in being leary over just grabbing a 2 output to use with the Z906 without certainty it can work first.
Of note, is that the Z906 is getting long in the tooth these days (sound quality wise and connectivity options - its capabilities were a bit more rare back in mid 00's and early 10's but is outclassed these days) and if you don't already have a way to support the connectivity options (coax, optical, 3x dual channels, etc), it becomes a lot less useful to hold onto them as you lose out on the main 5.1 aspect. And of course if you need any more decent or accurate sound producing options, the satelite speakers aren't the most ideal outside of gaming and some movies.
So your options in this case would have been:
1. Grab an AV receiver and use that to decode and then resend that over to the Z906. This isn't ideal as you would be required to add an extra piece of equipment to make it work as well as the cost, not to mention the extra wires. But it would allow you to connect many more devices at once to one device that then resends it over to the Z906.
2. Getting a different set of 5.1 speakers with more modern inputs. As mentioned before the sound quality of the Z906 is fine especially for gaming, but when compared to more proper (2.0 or otherwise) speakers, the Z906's speaker weakness' become very glaring. Many of the inputs for the Z906 are also of a (few) generations ago old now. So a new set of speakers might uplift what you hear as well as provide better connectivity for the future.
3. What you did already, grab a motherboard with the required outputs on them already. This would let you keep the current setup going without any issues for 5.1 to the Z906.
4. Getting a HDMI audio extrator active device (powered) that extracts the audio from a HDMI signal (have Windows send the audio signal via HDMI) and sends it via optical (from the HDMI extractor) to the Z906. This is a fairly cheap way to go about things to keep 5.1 but not all 5.1 forms is supported using this method. And of course, your resulting HDMI video output might be limited to 1080P (depends on the extractor, and HDMI cables in use as well, etc).
There's probably more, but I don't think there's really a cheaper option for you to take than possible number 4, but that will only work in the "majority" of cases, and not all cases. (Games for example likely won't work properly for 5.1 but movies likely will).
