Is there any variation in the vowel of the 3P suffix in Italo-Romance languages? In the North-East of the Gallo-Romance area where such epithesis happened, it's fairly obvious in the outcomes: the source must have been */-ntə/, with various dialects then changing the vowel: core Picard lost it entirely but the vowel still protected the suffix from total phonological erosion (amant > /ilɛ̃mt/) while Wallo-Picard and Central Walloon have moved the stress to the vowel and preserved it as an outcome of schwa, a short/lax front vowel variable by region (Binche: amant > /ilɛ̃mtœ/, Charleroi amant > /ilɛ̃mnœ/, Namur amant > /ilɛ̃mny/, Brabant amant > /ilɛ̃mnɛ/). Liège Walloon has /ilɛ̃mɛ/ with a /ɛ/ 3P suffix I'm not sure shares an origin with the rest.
If the languages of Italy systematically show /o/ (or a vowel that can be an outcome of /o/) then rather than epithesis, analogy with another verb form is much more likely (in the same way most of Gallo-Romance has a low/back nasal vowel borrowed from sunt as its 3P suffix (French is unusual))