Until now I dont believe many people have imported into TikiWiki. Well certainly no one seems to shout about it.
There is/was a phpWiki (another wiki engine) importer, but I doubt its been maintained and kept upto date with the recent changes in our wiki area.
We have a XMLRPC received wiki pages, and you can also import wiki pages through the mailin interface, simply sending each wiki page to a email address connected to the wiki mailin feature.
Personally if I was doing the job, Id take the exported file and write a command line based PHP script to import the pages into TikiWiki. That way you can easily use the wikilib and tikilib methods. Shouldnt be too hard to accomplish with knowledge of the Tiki code. But thats another story, our developer documentation is our code.
If your new to Tiki, you might want to seek out a TikiWiki Expert through FindTikiExpert and throw some $$$ there way to get the job done.
Its certainly possible tho.