Thanking you all for the great info in this forum...I just got a GPSMAP 62 ST (I wanted to buy a 62 STC but apparently Garmin are incredibly behind, at least in Europe as these new models are impossible to find).
Anyway, from another thread I downloaded and installed TrekMap Italy on a SD card with no problems whatsoever - just copied the GPMAPSUPP file in the Garmin folder and that was it!
Then I downloaded the city navigator for Italy and since that file also came with the same GPSMAPSUPP name I renamed it into GPSMAPSUP2, then just copied in the same folder of the same SD card, and it works fine with the device! So now I have both maps on the same SD card and it seems to be working fine.
I am in the process of downloading other maps and will try to name them GPSMAPSUP3 etc. to see whether they work too.
The only issue is that BASECAMP does not see the second map I just added, whereas it sees the first one with no issues - later.