It was a few days ago, with some help and reading other forums it's finally done.
At first it didn't work because the file webupdater.exe was in a different folder (C-Program files(X86)-garmin-webupdater, in windows7) so I copied the exe and language dll files to the garmin folder.
Then I created a mapset with mapsource covering all the countries I had from OSM (downloaded from here: http://garmin.openstreetmap.nl/
Good and FREE) and sent it to a SD card.
I created a folder named "test" in C: and copied the gmapsupp.img file into it and renamed to gmapprom.img.
As I'm not very good at computers and know NOTHING about command lines, etc. I created a text file in the desktop, copied and paste the command:
c:\Garmin\Webupdater.exe /upload_file rgn=49 path=c:\test\gmapprom.img -autorun usb /i
and changed the extension from .txt to .bat.
With the gps turned on and connected to the usb, gmapprom file in "test" folder, webupdater.exe and webupdaterlang.dll in the garmin folder, just double click in the bat file from the desktop...... a few minutes later:
Hope this is useful!
I'd like to say THANK YOU to all the people who shared this info in GPS forums.