It cannot be done through workspace unless you internally change how workspace sends data to the process. I think the workspace code that handles this is not accessible.
One way that comes to mind is to modify the rendering services for your initial step where you can add a service that can pull out the parameters data from some location like a database for example. But the question here is what if this data for these parameters is dynamic? Then in this case you must force the user to enter an id in the form that represents this combination of values
i.e id=0001 could represent startEndpoint=ApprovalSample.ApprovalSample&myExtraVariable=1234
id=0002 could reprosent startEndpoint=SomeSample.SomeSample&myExtraVariable=1235
So on your form you would have a dropdown list that would be populated with all these IDs (0001, 0002, etc...) through a webservice that pulls the ids from a database table.