Add 120 volt outlet to a 240 volt disconnect
I think what you actually want to do is install a subpanel. The subpanel can then include both 120V (e.g., 15A or 20A) breakers for receptacles or lighting and a 240V breaker for the original sauna use. The specifics will depend a bit on:


Existing wire size
Existing breaker size
Requirements of the existing sauna circuit


Keep in mind that there are GFCI requirements (which can be handled in the subpanel or possibly in the main panel) and that ground &amp; neutral should be kept separate in the subpanel. The subpanel could have a main breaker that is larger than the breaker in the main panel that is connected to the wire that goes to the subpanel.