AFAIK you can't really put a more detailed map inside a bigger one, unless you're willing to spend hours and hours on figuring out how the file format works and what to take into account.
One of the changes I made to Map This! involves automatically switching to the most detailed map for the current location. E.g., when you're in France, it shows the map of Europe, when you move back home to the Netherlands, it switches to the Netherlands, and in your home town it loads an even more detailed map.
See my signature for that.
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