[Zope] username restrictions

I'm trying to build a website where users from another website will log into mine. The other site has no restrictions on usernames. I'm running zope with CMF and plone, and the usernames for my website are very limited - no spaces or odd characters. Is the restriction there for a good reason? I imagine it is, but I don't know what the reason is. What I'm doing right now is the user must enter two usernames when he joins my site - the first one is from the other website, and the second one is unique to my site, and is used as the zope username. This is a hacky solution, and I don't like it, because all the good stuff that goes with usernames doesn't apply to their external username. Is there a better solution to this problem?