You can definitely do it in MG2 in the options screen.
I don't recall seeing the option in M2K/MG1, though; I'd check myself but my emulator messes up my connection. D:
It is possible, however to preate a perfect triplet in Music 2000:
Simply create your riff 3 or 6 bars long, and first 6 beats as your bar.
So instead of this (1/2/3 etc = beat. | = bar ending in 4/4.)
1 2 3 4 | 1 2 3 4 | 1 2 3 4
You now have
1 2 3 4 | 5 6 1 2 | 3 4 5 6
This results in changing the riff from 3 bars of 8/8 to 2 bars of 12/8; now just use every six 16th notes as one beat: 3 16th notes is now worth a quaver (8th note).
For a swing beat, simply go: (key: X = note. ~ = hold. - = silence. || = bar ending in 4/4. ¦ = beat ending in 4/4.)
As you can see, when you repeat it, it syncs up perfectly. The only thing you need to take care in is making sure everything syncs up fine.
Example you want?
NOTE: The song is now 50% faster than the tempo specified; if you were to put in a tempo of 080BPM, the song now goes at 120BPM. I had to change the tempo manually so it staying in beat.