I added mine to my quick text. I don't have my phone on me now, but it's in the settings menu. It's under your user dictionary.
Another option is to save everything before the
'@' and after the
'@'. Here's what I mean... If you're email address is
thisismylongemailaddress@gmail.com, when you type 'thisismylongemailaddress' it should have it listed just below that where your autocomplete would be. Long-touch it, and it'll save it as a customized word in your user dictionary. The problem is that it won't let you store the
'@gmail.com' portion because as soon as you type
'@' it removes it from where the auto-complete would be. However, you can save just the gmail.com portion without the @.
I recommend looking through the settings menu and finding your user-dictionary and adding the whole thing in there.
Hope I didn't confuse you...