Do you know if the hub has a multi-speed freewheel, or if its a cassette hub? A single speed freewheel will fit the hub of a mtb that had a multi-speed freewheel if that's what your asking. If you have a cassette hub you can get a kit which consists of a cog and a number of spacers to convert it to a single sprocket. With the single speed freewheel installed your chain-line may be off, in which case you'll want to adjust the spacers on the axle and then dish the rim to be centered in the frame again. If its a cassette hub with a single speed kit the wheel and axle wont need to be adjusted since you can move the sprocket to adjust the chain-line.