Glass packs?

i have a 50 series muffler on my car with stock piping, and you can barely hear it, i was wondering on what exactly are glass packs, are they mufflers themselves or are they welded on to the end of the muffler and are sort of like longer tips, i was looking into cherry bomb glass packs and was curious, would i be able to add the glass packs and keep my flowmaster 50 series?