Eurail does not count trips or journeys but travel days. On a travel day, you can use as many trains as you like, to as many destinations as you want.

You can't buy extra travel days to extend your pass.

No buying of add-on days.

BUt, do the math, if you discover it to be worthwhile you can of coruse order a brand new next pass for the nr of days you want.

Note that REGional trains in IT are still quite cheap and you may have to do a big lot of trips to even earn the cost of a passday-for the HiSpeed Frecce trains you must reserve+p[ay extra for all trips.

TrenItalia and some local regions also have much cheaper local railpasses for a few days.