The NMaven Visual Studio Addin has the ability to import Visual Studio solutions and generate NMaven POM files.
After identifying the modules, the Addin prompts you to identify which ones are NUnit Test modules.
References are converted to dependencies by examining the assembly info and checking the local repository for a match.
If no match is found, a system scoped dependency will be added pointing to the dll in the gac. (Is NUnit a special case?)
The Project Importer should skip modules it doesn't understand and print a warning message, rather than stopping.