Wayne, it's possible. As long as the 3rd party's software installs to the MAS90 directory, you should be OK. The simple approach on a Sage 100 Advanced would be to
Turn off the Application Server and Make a copy of the Live MAS90 directory first. (always the first step)
Rename the MAS90 folder to something else, for example OLD-MAS90
Copy the MAS90 directory from the test server - to the same place as the live directory
Go into the copied Test MAS90 folder and rename (to .OLD) the appserv folder in the /MAS90/Home/Lib (I think that's it) and I also reneame the activate.pvx in the \MAS90\Home\Lib\Keys (I not in front of my system, but I think that's it).
Now copy the same appserv folder and activate.pvx file from the renamed live OLD-MAS90 folder into your version with the 3rd party install and you should be able to work.