Suomi SMGs fire from open bolt. So unless you'e a class 2 manufacturer, you can't legally build one with the receiver sections offered.
If you make a new receiver that won't accept the original bolt, design a closed bolt firing system, and increase the barrel length to 16", you can make one work. It was possible for the Sten to be made into a legal semi-auto, so it should be possible for the Suomi. It'll just take a lot of work.
I'm mainly interested in the magwell. A drum fed, 9mm, 1/8" weld-pac AK would be way too cool.