Hi!
I just checked a few web pages on features and specs of an A37 and it turns out that it only features standard set of external controls found on earlier Roland A-series controllers. I didn't find any information on it having assignable knobs or not.

In either case you could always get an external fader box with assignable MIDI faders. I think JL Cooper still mnake them. They don't cost too much money usually...

-ED-