Unfortunately I don't think I can help you much. As you notice the work I did creating a way to import Wordpress content into Tiki is not related with what you are looking for.
Since you are looking to share the user base between Tiki and Wordpress, I suggest you check LDAP. I never used it myself. I know that it is a bit trick to setup LDAP with Tiki but it works. I don't know about Wordpress but I'm assuming that there must be a plugin or something.
You can start by checking Tiki documentation page on how to integrate it with a LDAP server:
Does anybody have idea about integrating(User registrations) TikiWiki with WordPress? Atleast a bridge is appreciated .
I need a functionality which my users have ability to access Tiki-wiki based site when registered/login to WordPress site.
Could anybody help me to do this?