Plugin for custom imports

Hi

I would like to import some data from our custom CMS over the years. All the import instructions on this forum and the Help seem to be geared towards importing content from other blogging tools.

Is there any plugin or another instruction set that tells us how to import stuff into the wp_posts table but also automating categories, tags, and authors?

We can do a simple script that does this, but I’m looking for the location of where this data should go.

Thanks!