You will pass through immigration between international airside to domestic airside on the ground floor of the junction of concourses ABC which is the opposite side of the terminal from where your LY flight will land.

Regardless, you will get better answers if your RBP your thread and ask that it be moved to the Thailand forum. This has nothing to do with LY in actuality. What you need to know really is whether you can check your bag at the transit desk in BKK. I don't know the answer.

If you do need to go outside to check your bag you at least won't have to go through immigration in the domestic end of the terminal and I haven't see the security line over there very long.

With whom are you flying to USM? Why not just check the bag all the way through? You don't need to be on the same ticket to interline.