Ehi Ricardo, glad to see your request
As @Stefano_Zamboni already said we just integrate Roundcube we don’t develop it. I suggest that you shoukd ask to Roundcube people, they will be happy to help you
Let me know once you make it work
just update the tinymce in roundcube to version 4.6.5 and it will work, except if you use Firefox. In Chrome and IE it will be pasted as HTML table, in Firefox as dumb image. I downloaded the newest tinymce from https://www.tinymce.com/download/ and put it together with the old one. I packed my updated tinymce directory and made it downloadable via WeTransfer.
Download tinymce.zip or tinymce.tar and extract to /usr/share/roundcubemail/program/js/
Maybe backup the old tinymce directory first by renaming to tinymce.old.
Nethserver Roundcube is version 1.1.9 with tinymce 4.1.10, newest Roundcube is 1.3.0 with tinymce 4.5.7, so I think the next Nethserver Roundcube update will fix this.
I hope I was able to help you…just let me know