adequickcopy,
Alas, I'm afraid you're right: PlanetPress doesn't natively support html format data. The only way you could use htm files in PlanetPress as data files would be to write your own HTML interpreter entirely in PressTalk. This would mean hours of work and a great knowledge of the suite, PressTalk and of the HTML language.
As for converting the htm file to another format, well, it all depends what format you want to convert it into. However, because we have no built-in HTML emulation mode, you would most likely have to use a script to convert it to a different format, and use your own data extraction methods. The closest file format to HTML that we support would be XML, though we also support other formats, like CSV, plain text, etc... Whichever is better/easier for you.
Regards,
Raphaël Lalonde Lefebvre