I'm not sure of what you are asking, but i think its possible
If you mean to code something that gets turned into from other elemenets automatically, well, that's going to be a royal pain. Since elements really only influence particles of their type, this would require more than just coding an element. LAVA and ICE aren't "being turned into" by their own code either. It's actually class Simulation that does the dirty work. Due to TPT's monolithic nature, said work is indeed dirty.
Ice is special in the way that for every element that it freezes into, a different element is created. The only one shown in the menu is ice that melts to water. The ctype is only changed to indicate to users what melts into what. Same goes for lava.