Double-dotted note within a triplet

I apologize if this issue has been discussed before, but I could not find the specific problem mentioned in the forum.

In the attached fragment, the three bottom voices should have one double-dotted 1/4-note and one 1/16-note inside the triplet in the first measure, so that all voices sing the last triplet-1/16 at the same time.

When I try to input a double-dotted 1/4 in the triplet, nothing happens. It's not a super important feature, but it would make this score prettier at least...