MMM is not a full DAW (digital audio workstation), and does not offer all of the features a true DAW (like Music Studio, Samplitude, etc.) would have.
MMM is not designed to allow multiple tempo's or time signatures in the same arrangement.
That said....it seems it is possible to "timestretch" a MIDI object (in MM-15) even though a pop-up says "that function is not available". You cannot "apply" the timestretch, but it seems to work anyway. However, it is not without issues.
It would be best to create a new MIDI object in the original tempo. Then, move or copy it to another track for modification.
The quanization grid does NOT adjust to the timestretching. Timestretching a MIDI object in a track that contains other MIDI objects can alter those objects as well. This happened to me when I altered one of four copies of the default Vita MIDI object in an arrangement with a tempo different than the default 120 BPM.
I suggest you create a "practice" arrangement and play with it to see if you can do what you want with it.