You can look at Messick's website and see what the parts would cost. I added creep range to my M6060 but it's a "simple" cassette arrangement so not particularly expensive.
I have no idea if this is your model but it's a start.
I would just make certain you are not intending to use the creeper gears for heavy work. The ones I have seem have a strong caution against it.......... loader work, pulling, plowing, pushing.
Dave