In the font "Opus Roman Chords Std", the glyphs exist for a third row of numbers, above the other two rows, but they are not copied into the "word menu" which appears when you right-click in a Roman Numeral text object.
The glyphs are available at the following code points in the font:
Digits 1-9: at code points U+00CE to U+00D6
# at code point U+00DC
b at code point U+00DD
natural at code point U+00DE
Digits 1 to 9 with strokes: at code points U+00EE to U+00F6
There are NON-SPACING glyphs, so you have to enter them BEFORE the bottom digit, which is SPACING.
If you are going to need these often, you should probably create entries for them in the word menu for Roman Numerals, in Sibelius Preferences.
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Sibelius 8.6/7.5.1/7.1.3/6.2/5.2.5, PhotoScore Ult 8.0.4, Dolet 6.6 for Sibelius, Windows 10 64-bit 16GB. Desktop no-name, Microsoft Surface Book.
With the legacy chord symbol text style, I can write any kind of chord symbol, Roman figuration, figured bass or combination of all three, but it's probably not for thee.
Well all three systems of chord figuration contain alpha and/or numerical characters. The chord symbol font contains Roman I & V as well as A thru' G and Arabic numbers. I have edited my chord symbol style to left of note justification and reduced line spacing. This depends on which font you are using. 50% line spacing works for Opus font and you can adjust default horizontal position. I prefer a bit to left of note, tho' this depends on your normal symbols width.
Others here can correct me but can't you just create a text for the roman numeral and create another for the stacked numbers? After each number you hit return within the text object and they all line up vertically. Make one of the two text objects non-magnetic so they can be near each other. Is that too easy?