You are correct - the weight maps will not be transferred unless there is a corresponding bone to match them to. However, if you are modifying a piece of clothing for a different figure to fit a current figure then you can use the fitting room or the setup room to transfer the rig from the target figure to the target clothing geometry. You do not need to copy groups. You can select which groups you want copied or you can group the mesh yourself to your own specifications. Ghost bones (helper bones) will be transferred - they don't need a corresponding geometry group to be transferred to. If you let the fitting room transfer it automatically then it will create random groups for each bone. I never do that, I'd rather create my own groups and leave that option unchecked in the dialogue.

Any bones that are not selected for transfer will not have corresponding weight maps in the target clothing, so it won't fit the designated figure as intended. The clothing rig has to match the figure rig exactly in order for it to follow completely. It can have additional bones if you need them for clothing adjustment, but it has to have at least the same bones that the target figure has.

A note however - you do not necessarily need helper bones to make up for a lack of jcms. It depends entirely on the mesh design and how well the figure has been weight mapped.