Pasteurizing frozen milk
There should be no problem with this from a food safety point of view. Freezing will "pause the clock" on spoilage, because bacteria's metabolism needs liquid water to happen. When you unfreeze, the food is as safe (or unsafe) as when you froze it. 

You should ensure that your thawing process is also safe. If you have the time, thaw overnight in the fridge. If you don't, do it quickly. For a liquid product like milk, stovetop heating will be probably best. Don't just leave it in the counter to thaw. 

I don't know if there will be some texture problems (such as clumping) if you cook or pasteurize previosly frozen milk. Try it out and see if it is OK. If yes, you can keep doing it.