a block with piston reaction like slime

you should probably poke about the pistons code / the code related to the piston - I looked up "piston" in the workspace file browser, used ctrl+f, and saw something slime related on line 112 of the PistonMovingBlockEntity.java file. I'm not invested enough to look into it further, but you might need to do some code editing, something like making the blocks class inherit from the slime block class instead of what it normally does or whatever.