any motorola shop can do it for you. if you can find the software and are skilled enough to build a pager rib and programming cradle you can do it yourself.
the one thing in addition to what quickcall6 said, it has to be in the same SPLIT of the same band. usually the splits are similar to 143.00-150.9975, 151.000-158.9975, 159.00-166.9975, etc.... so if it falls in teh correct split of the correct band it can be done. we had one local FD that got mad because their ch 1 was like 153.300 and the desired ch 2 was either 150.X or 159.X i can't remember now, you could not put them both in the same pager!