Solution: You can't. Gmail uses autocomplete.
Workaround: You can set up a contact and put what you'd like to be the nickname as the suffix.
Warning: Your recipient will see the nickname you're given him or her.
1. Go into Contacts
2. Click on the ellipses
3. This window comes up. Put your nickname, in parenthesis if you wish, under Suffix. Everything here will be visible to your recipient.
4. Save when you're done. It'll save automatically too.