Yes, you should be able to add a new module to SourceMods. Note that there are several subdirectories of $CASE/SourceMods/, so you want to put the file in the directory associated with the component that you are modifying. For example, you would put a new CAM file in $CASE/SourceMods/src.cam but a new CICE file in $CASE/SourceMods/src.cice.