Is there an inexpensive/easy way to convert the AES signal on the stagebox to analog to use as additional returns?
There might be a few options, I don't think cheap though. Someone else can jump in here, I'm not familiar with products like that.
The device you are thinking of is a Digital to Analog converter - something that is going to play a big role in sound quality, I wouldn't want to buy one cheap. Consider that one of the primary purposes you pay for on the stage box is the analog to digital converters on the 8 analog outs.
I still contend the easiest and cheapest solution is to use the analog to digital converters you already have on the back of the desk (the analog outs). A few analog runs paired up with your CAT5 run will snake them right to stage.
What other gear do you have that normally connects to the snake head? Anything that might have AES you didn't realize did? Anything that could be easily swapped for an AES carrying device at low cost? Just one stereo AES to your FOH speaker processor or amp , for example, would leave you 8 outputs in addition to FOH LR, which is very usable.