You can choose your own seed from the dictionary that electrum uses:
https://github.com/spesmilo/electrum/blob/master/lib/mnemonic.pyOther words don't work.
Electrum is great for offline use. You don't have to set your own seed to use it offline. Just run it as you would on an online system and it will generate a truly random seed for a new wallet. Much better than a human generated seed.