OK - this isn't a plug-in, but I couldn't find a more appropriate place to make this announcement.
I'm in the process of converting my users from SquirrelMail to RC 0.2beta. I wanted a quick way to move all their contacts from the old system to the new. So I wrote a Perl script that reads .abook files and turns them into rows in the RC contacts table.
If this utility would be helpful to you, read about it and download here (http://www.convoglio.com/roundcube)
There is also a script to convert .abook files to vCards at the same location.
Best Regards -- Steve
If there's someone still interested into this, I will just paste an historical copy of this I had in my archive. It's from about 3 years ago, but it still should work with minimal changes.
There is a plugin that converts squirrelmail addressbooks to RC.