Master Submixes, which are hidden from the user at the moment, are the only submixes in the Old Audio Engine. This was designed to allow for backward compatibility for projects upgrading to the new Audio Engine.
If you want some kind of Effect or DSP on a sound group, you will want to use the New Unreal Audio Engine.
For DSP on sound sources, you will want to use a Source Effect; for DSP on groups of sources, you will want to use a Submix.