On your system you have the following folder :
/var/cache/apt/archives
In this folder you will find most debian packaged previously installed. Among them there should be :
openmediavault-transmissionbt_0.3.6~1.gbpc96991_all.deb
Or an earlier version. Then you can install it just as any plugin. Be sure to first uninstall the previous one.
You can also try to download the previous plugin somehow but i don't know where the repository is exactly.
The above is just an hint of how to do it. Please don't do something if you don't know what your actually doing