Kate, there is a check box for Import HTML so it appears that you can import pages from another source that are created with HTML, and it is preserved as such on Tiki.
But, I am not sure if you can make the entire site HTML by default.
Honestly, if you import the WP pages as HTML, and keep them as they are, then from now and forever use native tiki syntax for the rest of your work, I think you would be better off, the beauty of syntax is search engines love plain text formatting. Search engines give higher priority to plain text, and with the tikiwiki WYSIWYG, it's not that far of a leap to have the benefit of a familiar word processing editor that generates wiki syntax. I think your site will be better off in long run for it.