Figured bass with dotted rhythms

How do you get a figured bass rhythm group to last a dotted duration? I'm writing a piece in 12/8 and the groups variously need to last for a dotted crotchet, minim or semibreve, but MS seems to only let me change it to be an undotted note value.

The only way I can figure out is to write a bass line with one note per group, add the figures, and then change it to the actual bass line; but that involves deleting the bass line that I've already entered in full.