I was just thinking about this last night. While I do like Montreal steak seasoning, I find it too course, and could use a blend. I had a bite of steak and got a large hard chunk of something that was off putting.
Of course, this is if I’m interpreting your question correctly.
Just put it in a spice/coffee grinder. I do that with all of my steak rubs. At home I mainly use SPG or Boars Night Out Midwest Dust, but for our competitions we use three different commercial rubs. DB180 all purpose, DB180 beef and Boars Night Out Double Garlic Butter. All ground.