There is a tutorial here that shows how to do some modification to the
Blog Module. You should be able to use the same techniques.
MIchael Washington has some nice tutorials on module development
here. There are also some tutorials at
DNNWiki.
What you'll need to do is to set up a development environment that includes a copy of DotNetNuke installed so that you can open and debug it with Visual Studio or Visual Web Developer Express. Then, with DNN running, you can install the Repository Module and begin to modify it's code. When you've got thing running the way that you need, you can then extract the modified Repository, package it in a zip file, and install it on your test site.