one way of doing it is to preview your tx before signing it, then copying the hex and editing the amount field to include the fee in it (increase it +fee) and then giving it back to electrum to sign. of course you mustn't do it without knowing what you are doing or you may end up messing things up.
Do not manually create or manipulate Bitcoin transactions unless you absolutely know what you are doing (and think twice even then).
You don't want to end up as the guy spending 10 BTC in mining fees on a zero BTC transaction...