If the shut off is screwed onto the pipe that comes out of the wall and the shower head is screwed onto the shut off, then yes, you should be able to replace the shut off valve with another one. I've purchased chrome plated brass ones with a push button valve at Home Depot before.
On edit: If however, the shut off valve is part of the shower head assembly and not separately removable, you still may consider adding the part I described above, and leave your integrated shut off valve fully open.