No you can't, at least that's how it appears to me (after looking at the Gmail app on 3 different Android devices). You add a contact using the Contact app, and fill in the email address. Probably should make sure that it goes into the Gmail list rather than some other one.
BTW, I also looked at Gmail on my PC, where it's also called Contacts.
If you just want to enter the email address somewhere without creating a Contect item, then create a Draft message with it in. or use a Memos app or something else. I would probably create a Contact but otherwise I'd probably use Evernote.
Steve