This question is already answered in the Electrum tutorials http://electrum.org/tutorials.html#switching-to-electrum

> **What to do with my old addresses and wallet when switching to Electrum?**
>
>The best way to switch to Electrum is to send all the bitcoins you have on your old wallet to one of the addresses on your Electrum wallet. This way you'll have all your bitcoins secured with your seed.
>
>You could import the private keys from your old client, but you will have to keep a backup of those keys separately as they won't become part of your Electrum seed.

Remember not to totally delete your wallet.dat if you gave your addresses for the deposite to other people