It can be done, but it would be manually. If it didn't have to be sinusoidal and you just wanted it smooth, you could just average the old and new motor speed values. I would suggest about 3 to 1 average, old value to new, such that you would have code like this.
oldMVal would and motorVal would just be standard character variables. You would change motorVal in your code to run the motor smoothly. I think the (int)s would be required to keep the values from overflowing.
Anyhow, this would do it, with just a short pause between loops. Maybe like 50 milliseconds. More if you want it even smoother.
If you really want perfect sinusoidal control, well, RobotC does have some built in trig commands.