If you want to add a datafile you can do as joe writes or through SSMS.

Is there a specific reason why you want to move tables to a different datafile?

If you add a datafile (.ndf) as I understand it the data are spread across the datafiles evenly when you make a rebuild of all the indexes or when adding data, correct me if I´m wrong.

If you want to determine the fill level of the datafiles before/after a rebuild use the tips Here.

Regards

Stefan