Well... Since it's a totally blank file, you need to win some cars for it first - you can't just plant extra cars into a garage. You could, I guess, use the 304 editing to give yourself a stupid amount of money and buy a few hundred Cuores, then replace them.
All the cars you put into the file should have in the .ini file a "US" region code - the programme won't let you put any others in. The game will stay in km/h regardless.
Let me know how it works out with the F1 cars - I'm curious what happens this way round. NTSC-to-PAL it just converts all the F6nn/x to a P002 and all the F0nn/x to a P001.