You probably won't be able to interface with any of your buttons on the actual card via the front end unless you can configure Girder to do so. I have never played with girder so I can't say yes or no.
If you are using a frontend such as roadrunner that utilizes winamp, then you can get the vst plugin bridge (also a wiki article explaining how to do that) then you can load an EQ from within the vst bridge, look for EQ vst plugins. There are a few listed in the mp3car wiki pages under audio tuning.
I can't tell you which EQ does compression limiting so you will have to look for yourself. There are literally all sorts of VST EQ's available. We have only linked a few of the more popular ones and or free ones.