Yes
Here it's explained
http://wiki.openkm.com/index.php/Create ... extensions
Take a look how to configure developer ide at
http://wiki.openkm.com/index.php/Developer_Guide
And in OpenKM trunk there some minimal examples at com.openkm.extension.frontend.client package
I'll create a section about extensions on developer forum
I suggest first be familiarizated with IDE, debugging etc.., then understand the examples and run someone, after it try building your own. If you need accessing some UI data not present in Comunicators, tell us ... we're extending it ( we've started with what us are considering the necessary, but sure it'll be extended ).