It sounds like that the WSP you have there is the result of one choosing the "Save site as template" option in SharePoint 2010.
This creates a WSP with a single Elements file that contains one hell of a lot of information, including a sort of Site definition.
You will be able to poke through this file and extract the elements you required into separate Element files in a new Visual Studio Project as you see fit. This is a recommended approach.
The Import WSP file just does not work properly when you import WSPs that were created with the "Save site as a template" option. You have no further benefit than if you renamed to .cab, extracted then opened the resulting files in Notepad.
Does the WSP contain the workflow defnition element in there somewhere? If you say it comes with one, it should be in there somewhere. If the workflow came with a binary assembly you'll also be able to extract it and include it in your new Visual Studio 2010 project.