I don't think so, but maybe one day if some enterprising person does it or we get some help from Arturia.
The firmware download is a zip-compressed archive of an ordinary JSON file and a couple binaries, I guess for two different chips. Looks like a STM32F4 chip for the "main" binary. Doesn't seem to have been engineered to thwart end-user modification, it's certainly not encrypted, though make there's some signature validation that I can't see in a hex editor here.
It would be a massive amount of work, though. Patching the existing firmware would be difficult enough, custom firmware would require writing the entire thing from scratch unless Arturia releases source code or some kind of SDK.
It seems like they don't intend to support this product anymore based on their last firmware update + lack of activity in this forum which is disappointing as a new, mostly happy owner of a KeyStep Pro. Kind of makes me feel like I've been left holding the bag given some of the complaints I've read here. But if they make it easy for us to make our own firmware I'd be happy to be one of the first to try.
I might not try unless I hit some workflow-destroying bug, it's a massive amount of work to do it without help and I wouldn't want to risk bricking it.
