You'll want to change their animation, I think. (I'm a fiddler, not an expert.)
1. Open your NPCs visual resources (where you change their clothes and hair and stuff) and look through the animations listed in the drop-down above the preview window.
2. Available animations vary a bit depending on the root template you're using, but you should have at least one or two options that start with "training." Make sure the word "loop" is in the name somewhere, and copy the name. Close the window.
3. Open up the Scripts option in the sidebar for your NPCs and search for "anim." Pick one like GLO_LoopAnimDuringPeace (feel free to play around until you find one that behaves how you want) and paste in the animation name you copied earlier into the correct field.
4. Save and reload.
5. You might need to get your characters to draw their weapons first. I don't know if they're drawn automatically when you use an animation with the correct weapon type, but I doubt it. There's probably a script for that too.
