I have not studied the file formats for both electrum and the bitcoin-qt clients but my guess will be they are different and you cannot just copy the electrum.dat file to your wallet.dat file for Bitcoin-Qt.

Here's an alternative way to transfer the bitcoins. You can send the bitcoins from the Eletrum client to any of the addresses in your Bitcoin-Qt client. Sure that will put the transaction on the bitcoin network but at least you will not be meddling with the wallets directly.

HTH