With loops you are going to have to match tempos, using either time-stretching programs, such as Acid, or beatslicing, as in Propellerhead's Recycle. The sound isn't as good as Recycle, but IMO Acid (Screenblast version) is probably the easiest way to do this, and the least expensive (about $70). The only problem is that Acid does not "talk" to nTrack (no Rewire Slave support), so unfortunately, you have to arrange your drum loops in Acid, export them to wave files, and then import them to nTrack.
Another option is a VSTi sampler with individual drum hits played with a MIDI controller.
I would tell you what I use, but it's become too complicated, and I don't want to rant!
Man, I wish I could play drums!