Yes, sew on a webbing tail of whatever fits through the current buckle.
Those locking tri-glides work great for lots of stuff, but I think the largest size they come in is 1″ (I could be wrong). I’m guessing the belt you want to use is 1 1/2″, and you need to shrink that down to 1/2″ or something to connect to the buckle on the pack? The most solid connection would be a webbing strap sewn to the end of the belt you want to add using the existing pack buckle. If you have a sewing machine and some scrap webbing, this is free, 10-minute job.