Hello,
Currently we have no plans to include and maintain any custom plugins for Roundcube. It should be possible to install plugins for Roundcube on your EPESI installation, the Roundcube itself can be found in modules\CRM\Roundcube\src folder.
If you mean to try and install a plugin to Roundcube, it'd be great if you could let us know if there were any difficulties related to EPESI integration.
Kind regards,
Arek