The Tempown power allows main owners to make owners for a fixed amount of time.
To make someone a temporary owner, private chat the user and type /mo? whereas the ? is the number of hours you'd like them to be an owner. The maximum amount of time you can make someone a temporary owner is 24 hours. Temporary owners can not ban longer than 24 hours, unless the chat's Gcontrol settings are set otherwise. They also cannot make permanent moderators.
Type /mo12 in someone's private chat to temporary owner them for 12 hours.
Temporary owners are able to make temporary moderators if they have the Tempmod power.