I don't believe your filtration will accommodate filter bags without making some modifications to your sumps. If you want to add some mechanical filtration, you can try some 100 micron filter pads. I know the blue "prefilter" stuff you are talking about and it's garbage. On my old wet/dry truvu truflo I used 100 micron filter pads (
http://www.marinedepot.com/ps_viewi...campaign=mdcsegooglebase2&utm_content=UJ00126) and they worked well. You can also get finer pads, but they will clog pretty quickly. I'm not sure what would happen on your setup if the pad starts to clog (on my system, the water would just overflow into the other area of the sump with no problems), but you would obviously want to make sure that the water has somewhere to go other than over the top of your wet/dry filter and onto the floor.
To clean them I would just lay them on some cement in the yard and spray them down with the hose on "jet" mode...worked really well to get all the crud out. I could reuse the same filter pad for over 1 year.