If you edit the loops in a DAW you can load them right into Rytm, as long as you keep the Rytm at the same bpm as your edited sample.
If you want you can experiment with AR pseudo timestretch techniques:
-Timestretch
An OT would probably work for you and most likely provide all sorts of new ideas to utilize in your music, but I already sense resistance in your posts; wanting it to be easy, painless, fufill all of your wishes, etc… The OT won’t like this attitude, it requires you to work a bit when learning it, but will reward you with much more than you expected if you put in the effort… It’s not as difficult as people make it seem but still it’s much better to go into it with an attitude of “OK, I’ve got a new complex machine to learn and after I do it’s going to be creative and fun for my music” than “OK, this machine will make my music creation easier”…
With practice it’s easy, in the beginning it’s not…
I second @jb 's idea of uploading a loop and see if someone will timestretch it to a desired tempo so you can see how it sounds. Even though you want to use many loops you could still get a good idea of the algorithm by hearing one as an example. I’d do it but I’m not with my OT. There are two settings for the timestretch, Normal and Beat, and there is a transient detection knob. It’s quite possible playing with those settings could get you better results, but hearing some kind of example would get you an idea of the sound…