The URL structure is not customizable at this time. Getting things like forum titles in there is tricky, at least to ensure the data is always available to the link builder. Not to mention the obvious possibility of conflicts, given that all the URLs are in the <prefix>/<action> or <prefix>/<content>/<action>.
It's a little hard to explain, but it would be possible to build a URL scheme on top of what's there, though that hasn't been tried yet.
I do have a 301 redirector for vB3's showthread/forumdisplay/member.php links, as I needed that to convert my board, but that's sort of a different issue. Having "support" for imported content URLs is a nightmare, particularly if they conflict with our URLs.