I haven't actually done this yet (only just saw this thread today, which is handy as I'm going to the States from the UK on Thursday and would like all these maps offline)....

Bratac - I'm guessing that you're misreading the message windows is giving you when you try to paste more than one map pack on to your phone. As the folder names (1, 2, 3 etc) are the same, regardless of map pack you are installing, windows will tell you that you'll need to overwrite... Thing is, you're not overwriting anything - on Windows 7 I think they actually term it "merging" folders, which is all you are doing.

As an example, Spain will have a folder "1" with a file "Spain.map" in it. France will have a folder "1" with a file "France.map" in it. Say you've already followed the guide to install Spain and now you try to extract France, you will get an overwrite prompt. If you click yes or yes to all etc etc, you'll end up on your N900 with a folder "1" that contains both France.map and Spain.map...

Let us know how you get on - sorry if I've missed the point of your post but think this is what you were getting at...