Sure
I would suggest using 3.01.14 as the starting point, there's a lot of good new stuff in there and the process is simpler.
IMPORTANT NOTE:! 3.01.X is still a .Net 1.1 assembly and as such can only be edited and recompiled using VS2003.
Pre-reqs:
1) VS2003
2) Working 3.X DotNetNuke web site
3) Nant (optional: for repackaging)
Preparing your development environment:
1) download the 3.01.14 source package
2) open IE and run your DotNetNuke 3.X web site
3) log on using the Host account
4) go to Host->Module Definitions
5) Install the Repository 3.01.14 Source package you downloaded the same way you install a normal install package
6) exit the browser
Modifying the Repository module code:
1) Open VS2003, open the /DesktopModules/Repository/DotNetNuke.Repository.sln solution file
2) check all the project references to make sure they're correct, fix any that are not (they might not be correct if your DotNetNuke web site is not is the default location of inetpub/wwwroot/dotnetnuke
3) rebuild the solution, hopefully it wil build cleanly the first time
4) now, you can debug the code, modify it, test, etc.
Re-Packaging the module for distibution/installation:
1) rebuild the solution in Release mode
2) exit VS2003
3) use Nant to execute the build file, DotNetNuke.Repository.build
When done, look in the /DesktopModules/Repository/package folder, you'll see two new .zip files, one install pacakge and one source package. You can now take the install package and install it on ANY DotNetNuke web site, either 3.X or 4.X.