My budleia tend to self-seed here and there, but then that's not going to be where you really want the shrub, right? Looks like you can propagate from soft-wood cuttings, so the new shoots should work fine. Just do the usual: snip them off and remove lower leaves. Dip ends in rooting hormone if you like, place in moist soil in a pot. I have not tried this but I think it should work. The earlier in the season, the better. Let us know if you have any luck!