See module in action here

DNNMasters UserSwitch is just what the name says - a SU-like utility for your DotNetNuke portal.
Easy switching of user context without the need to log off and log on again and again.

A must-have tool for any DNNMaster, Administrator and Host

Security settings in Options prevent users granted module access from switching to admin role. Super User (Host) role switching is denied to all by default.

Module is available on www.snowcovered.com