I'm guessing you mean the 16 tracks-- really MIDI channels-- which let you use up to 16 different sounds at once in a song. If there's a need to use more than 16 sounds in a song, the usual procedure is to change the sound on one or more of the channels. For instance, the song might start out with an acoustic piano sound on channel 1, then at some point in the song it might change channel 1 to an electric piano sound, then later it might change back to the acoustic piano sound, and so forth.
Is this for live performances, or for recording songs? That might make a difference in choosing the best approach.
If you're talking about live performances, you can save the setups for the 16 parts (or channels) in a Performance, and then if you need to use different sounds at another point in the song you can recall a second Performance for that portion of the song, then a third Performance, and so on.
Note that a lot of gigging keyboardists are moving toward using an iPad or other type of tablet to help manage set lists and transitions between different sounds in each song.
If you're talking about recording songs, you can use the keyboard's song editing feature to change the voice on a given track at a particular point in the song.
If the keyboard's song sequencer doesn't have that sort of editing capability, you can load the song into a DAW or other MIDI editor on your computer, make the changes there, then save the edited song and load it back into your keyboard to be played back.
I'm not familiar with the Motif's features and functions, so I'm not sure what your best options are, but if you can give us a better sense of exactly what you're trying to achieve then the forum members who have experience with the Motif series should be better able to make suggestions.
