If you mean : is it possible to control a motor from an Arduino board and power the motor from a seperate powersupply, then the answer is yes. Actually with most motors this is required since the Arduini board can only source a very limited amount of current. Muc less than most motors will need.
Check this example : http://www.tigoe.net/pcomp/code/category/code/picbasic-pro/62
Its not Arduino but the setup is exactly the same for Arduino use.