This is tutorial how to do this

1.) Download original unlocked img for AR 4.20
2.) Download unlocked img from Snecer post #7 where is in img only Slovenia 4.10
3.) add both img on device or SD
4.) on nuvi we enable both map ( AR 4.10 Slo and AR 4.20 )

Now you will have working AdriaRoute 4.20 with all country.

Snecer has done great job