Imagine a 3/4 bar with three quarter notes in it. I want to change these three quarter notes to a quarter-note triplet and tie the last note of the triplet to a new quarter note to fill out the bar. How do I do that?
Select the notes, press ; for the tuplets popover - it will probably already suggest the ratio 3:2 but if not, enter 3:2 and press Return. Then select the last note and press Shift-Alt-Right arrow until it’s the required duration.
Select the first quarter note, invoke the tuplet popover and write 3, enter. At this point, you should have a triplet of quarter notes. Next, you can use alt-shift-right to make the last note longer, or input another quarter and use t to tie the last two notes. Easy peasy
Edit: Lillie was faster 
Thank you very much!