I don't believe there is an out of the box way in Jive to do this (not that I've come across in the Cloud version, at least).
I did this in our external community for a few execs. We use SSO for our community logins, so I was able to log in as them through our SSO service and do any necessary actions.
Hi Justin Adams,
While there is not an out of the box option to post as a different user, our Professional Services team has developed an add-ons that will accomplish this. For more information on the ghost blog add-on check out - Delegate Access (Ghost Blog) FAQ - Archived because of publishing issues