Hey Raionnyan,
if by plug-in you mean the standard packages, RV comes with some testing packages, that you can modify.
For examle the Python Example Mode and also the three below in the image.
If you want to test them safely, copy the package into the path:
USERPROFILE\AppData\Roaming\RV\Packages (in windows you can enter %appdata% in the file-explorer and it will take you automatically into USERPROFILE\AppData\Roaming)
You can also rename it.
Then your package will show up in the list of the picture above and you can install and load it.
The python file (or files, depending on the package) will be copied into
USERPROFILE\AppData\Roaming\RV\Python, where you can edit them without changing the package itself. You could also just copy the python file, if you want to change an existing package. All the changes will only be for your user.
C:\Program Files\Tweak\RV\plugins\Python
here are all the python files saved into, that come with rv. If you need ideas on what to code, you can check them out too.
Hope that helps.
Cheers, Henna.