I would like to open and modify some surface models that are being created in NX using SolidWorks. What is the easiest way to save them in NX and open them in SolidWorks with all the features and editable parameters?
You can't.
Importing models into solidworks will leave you with a "dumb model". It won't have a feature tree.
You can add more features to the imported model, but you won't be able to modify the parts like you could in NX.