Back in 2014, forum user @nadseh wrote a little ModuleManager script to add MechJeb and Kerbal Engineer to any vessel with a command pod. It hasn't worked for KER for a while, and he hasn't updated it, so I've adopted it and released an updated version. The original thread is here: https://forum.kerbalspaceprogram.com/index.php?/topic/75295-0235-mechjeb-and-engineer-for-all/
This now adds kOS to all, if you are using kOS
Note that the KER functionality is also available with KER its