From what I'm seeing in the F-20 Tigershark schematics I find online, it'll be quite troublesome to put in internal bays. It's got a pretty flat bottom, and it's pretty small to begin with, so I don't think there's space for an internal bay unless you bulge out the underside, which might cause aerodynamic problems due to increased induced drag. I guess if you really want additional hardpoints, you can try putting two more fore of the fuselage landing gears. They'll only be able to carry short missiles due to space issues, but it should work. Admittedly, I don't have a weight-and-balance diagram of the F-20, so it might cause some instability issues.