Workingmans got loads of interesting variations in it already, and swinging door needs just to be solid, with those passing notes as others have said.
My advice as someone who knows no theory is learn the basic bass pattern and the structure of the song, once you've done that that listen to some of the variations and learn those if possible , but don't attempt to add anything that's not already there ( unless the brief for whole band is to do it differently ).
Groove is always talked about with soul and funk, but I think this type of music whilst simple enough, can be ruined easily by over playing and losing time, it's all about the groove on root and 5th
Good luck, they are great songs