If a player has a Nydus Canal, can he send units other than zerg units into it?

Test this yourself. It would take less than 5 minutes....

But the answer is no.

No. Are you trying to invoke a teleporting system of some kind into a map? I'd be more then happy to give you some suggestions if you really need something similar to a nydus.

uhhh, yes

Well, if you need some help or ideas feel free to ask, but give some information about your map as you do so - that way I can help to best visualize the most appropriate set of triggers for your specific needs.