It is but.......
If you do not have the WTE (World Traveller Edition) and I assume you don't, or you wouldn't be asking
) ), you only have the 2-gig version of the 540.
So...........
You'd first have to use Explorer to copy the USA_Canada_Mexico map from your unit to the computer. Then, after making sure the backup is on the computer, delete the map folder from the unit.
Next.........
While you can purchase maps of Europe, your unit isn't large enough to store the entire Europe map, since it is > 2.2 gb in size.
So...........
When you connect to Home to make the purchase, you'll be asked what Europe zoned maps you want to install on your unit, for example Europe_West.
The remaining zones will get installed on your computer. If you want to switch zones, you'd go to Manage my Unit, Computer, Maps and expand that section. You'll see a new link there to 'change zones', and if you select that, a different European zone will get installed.
In summary:
Yes you can get Europe maps but you need to remove the current map and then select a 'chunk' you want to install.
Finally, let's say you do go through all of this and install, say, Europe_West, but you don't need it for a few weeks. Then use Explorer to copy it to the computer and use Explorer to copy back USA_Canada_Mexico for now.
Convoluted, eh?
But it'll work........