If your question is how to contribute code (as opposed to what areas),
then the formal answer is by filing a change-request (or other type)
PR with "send-pr", and include the patch. Informally, it might be
appropriate to go through some NetBSD developer who is active in the
area of code that you're working on.

If there are potential design issues to be aired before writing the
code, you'd do well to dsicuss them on the relevent tech-* lists.

--jhawk