I’m trying to figure out how you would use a linear module as a B axis. I thought A, B, and C axis were all rotation about the X, Y, and Z axis (respectively)?
I’m pretty sure you’d have to modify the firmware to accomplish anything though. The firmware is available, but I’m not aware of any effort to build and package it as something you could install on the controller. This is probably the closest thread I can think of, and includes a link to the original firmware source. Although I think this thread has the latest source. The source for the v1 didn’t get the same love the v2 received 
I know the 2nd thread started out as a v1 thread, then was later moved to the v2 channel.