I am looking to modify the way the plugins work and to add some additional functionality. Is there a copy of the source code?
yes is shared but we use add-in express installaer ( without it ) you'll get some compilations errors ( althought it you could send to us some changes and we could try appy.
http://openkm.svn.sourceforge.net/viewvc/openkm/trunk/
MSOpenKMCore/ the main libraries and the others are obviously
The idea is changing MSOpenKMCore, for it you don't need add-in express support
Take a look here too:
http://sourceforge.net/scm/?type=svn&group_id=180733
http://wiki.openkm.com/index.php/Developer_Guide