Map NL,DE,POL

Hi, I'm planning a tour from NL to Polen. And my old edge 130 just broke and i'm looking for an upgrade. Now i saw the 540 and I think it would suit me. But I read that the internal space isn't that huge. I saw some pictures/screenshots of the way of adding maps, like west/east/central europe. But they all missed one of the countries I'm going. Is there a way to add only the countries I want. Or do I have to upgrade to the next up 840 to get the coutries I want and the others with it and just wasting the space. Thanks in advance!