This is possible if you load your c# add in into a different app domain in microstation.
By default microstation will load your c# add in into the microstation's global clr domain.
Try these commands:
MDL LOAD microstation.plugin.cant,,microstation.plugin.cant
clr unload domain microstation.plugin.cant
My project and dll was named microstation.plugin.cant, so you'll need to change that round to match whatever your c# dll is named.
There is a good guide here:
The rest of that blog series is really handy to have a read through if you're developing c# plugins for Microstation