Can I make my crumble topping in a mixer?
You could use just about anything to mix that, a food processor, a mixer, a stand mixer (just about any attachment), even a blender would work in pulses. Freezing it too will work just fine. Just move fast because any of those methods will create heat, and heat will melt butter. As long as your ingredients are mixed and your butter (margarine) isn't melted, you're golden.