I think the best way to do this is to use the Chord Tool. This will take awhile to set up initially, but then it's ready to use from there on out.Here's how:
Select the Chord Tool, make sure that "Manual input" is selected under the "Chord" menu, then click on a note. In the dialogue box that comes up, make sure the Advanced Options are visible so you can see the "Select" button next to "Suffix." Click this button to bring up the selection of suffixes available.
At this point, you can create a new suffix or edit an existing one. Take a moment to view some of the settings of the existing figures, especially the ones that have smaller numbers stacked vertically, as you will want to do, for example, for Tonic 6/4 chords.
When you have the Roman numberal as you want it, save it.
When you use these, make sure that you de-select the option for showing the root of the chord. All you want to show is the Roman numeral figure you created as a Chord suffix.
One more help: You can program a metatool to make accessing your figures very simple. Here's how:
With the Chord tool selected, hold down the shift key then type any letter or number that would be easy to recall. (For instance, the letter V for a Dominant chord.) Then, whenever you want to use that figure, with the Chord tool selected, hold down that key and click on the note (or chord) that you want to attach that symbol to.
This is a lot of work to set up, but the good news is you can save the set up as a library that you can then use in other documents.
