Multiple Roles?

I’ve been playing around with various ways of handing out specific permissions to members (and semi-staff) of my WP blog, as to limit the areas where-, and how they can post. However, I’ve ran into one problem. I’d like each member to have one (sub)category where they can blog themselves, after having had their posts approved by “moderators”. This wouldn’t be so much of a problem, if I didn’t want one (other) group of members to be in charge of said subcategory’s parent category (the so called “moderator” group).

Now, the whole restriction thing was solved with the “Limit Categories” and “Role Manager” plugin, but that only works for one of the situations at a time, not both. If one user were to be a “moderator” for one category, that user wouldn’t be able to have his/her permissions limited elsewhere at the same time (being able to publish his/her posts under the category assigned to him/her for moderator purposes, while at the same time having to have other posts approved by other moderators when posting under other categories).

Now, in theory, I would be able to solve these problems if I could assign two Roles to the same user, seeing as that would enable me to give the user different access levels (using Role Manager) depending on the category they post under (using Limit Categories). Is there a way I can do this, or can I solve my problem in another way?