The Bozak uses summing resistors, with the mix-amp having a high-impedance input. In other words, you can easily add a stereo channel (as Dave says) - and you could probably add another stereo channel or even more without an issue (I haven't tried this yet - every extra channel will increase the impedance seen by the mix amp - my guess is that you could easily get away with a pair of stereo channels replacing the mic channels without any issues).
With the CMA 10-2-DL I'm working on at the moment, I've replaced the original Mic pots with stereo RK27 pots (I have too many in stock!). If the user wants to add a stereo channel, they can connect a jumper straight from the rear input to the pot. You'll need a second summing resistor, and voila, your first Mic channel becomes a stereo line input.
Alternatively, you could connect each channel to each Mic pot and Pan L/R accordingly.
You could replace the 3-pin XLRs with 5-pin XLRs for a stereo signal - this could be easier than adding new Phono jacks.
The biggest issue you're going to have is the fact that there are no cue-ing provisions for the Mic channels… I told my current client that I could convert the Mic channels to Line channels easily, but he went off the idea when I told him we'd need to replace the rotary switch and re-print some silkcreen on the panel...
Adding a raw stereo input in place of a Mic channel is easy. Getting a cue feed is not so easy... Decent rotary switches are an endangered species btw - a good one will cost a lot unless you spend a lot of time looking for surplus on Ebay.