EZdrummer does include some 3/4 grooves.  6/8 too, which will fit into 3/4 time… they’ll just feel a little fast.
7/8 is a bit unusual.  I know http://www.oddgrooves.com/, for one, does have grooves in 7/8 time… there’s a few in their free demo anyway.

I’m not entirely clear if you found the time signature adjustment in Reaper.  If not, it’s right next to the tempo indicator.  Adjust it to match the time signature of the grooves you’re dropping in, and they should fit.

But yes, dropping a 4/4 groove into a 3/4 time signature isn’t really going to “work.”  It’d be like dropping the proverbial pegs into holes.  With a round peg through a square hole, assuming the round peg’s diameter is small enough, it’ll fit… but it’s just not the right thing to do.

You can edit the grooves within Reaper and cut them down a quarter beat (for 4/4 to 3/4), but in doing so, you may notice the “feel” of the groove is off when played along with the other trimmed pieces.  Or it might work just fine.

Some other expansions that include 7/8 grooves would be…
– Vintage Rock EZX
– New York Vol. 2 SDX
– Custom & Vintage SDX
– and of course the Odd Meters Monster Midi Packs

Additionally, in Reaper, if you want to mix time signatures, you can right-click on the track and “Insert Time Signature Marker…” (Shift-C).  Using that, you can start a song in 4/4, move to 3/4, back to 4/4, go crazy with 7/8, and then go off the deep end with 17/8 or something weird like that.