h2pas comes with full sources including a grammar. You will just need to change the grammar
and feed it through plex/pyacc, then recompile.
You will need to resolve the ambiguity you just introduced by hand, though. See lex/yacc documentation on how to solve that.
When you've done all that, plz submit a patch....
It is probably enough to change just the lexer part, although you will need to add a name generator to keep the rest working. (Also useful for nameless parameters, that are also not supported)
/utils/h2pas is the sub-directory
I have looked at it and it should be doable for beginners that invest a little time in lex/yacc examples.