Quite a while ago I posted a solution on how to enter quarter note triplets in chords (and the bass and drums).

It is by dividing a 4-beat bar into;
one 2-beat bar
and a waltz (3/4) bar.

The waltz bar has to be speed adjusted in order for it to sound like the same tempo as the rest of the song.

Please try to find my posting, as also the calculation for the tempo adjustment was there.

Brgds,
Tonnie