What happens if a transfer is made to a unused address from the keypool? The bitcoin-qt client generates 100 addresses from the start. But they are not shown in the "receive"-list (only one).

If I export such address with pywallet and send something to it. Will it be recognized?
As long as the key is mathematically valid, which it will be, then it will be recognized.  The blockchain simply keeps track that someone sent coins to that address. It doesn't need to know anything else about it--the address isn't "registered" somewhere in advance Your wallet will contain the private key for that address which means that you will be the only one able to receive and spend the bitcoins which belong to that address.