There are many extensions that would benefit from being supported by DOCman out of the box. There are however some problems with that:
There is a better solution available. DOCman has a number of events, which allow 3PD's to create plugins. These plugins hook into DOCman to add functionality, the same way plugins can hook into Joomla! itself. With some creative programming, it's possible to do almost anything. When you want to do something that can't currently be done, you can create a patch for DOCman that adds a new event. We'll be happy to include the patch in the next DOCman release.