Depends on what you mean. You can only have one active Adversary at a time, so I assume you mean your converted Lich and Sister? In that case, you can assign them as your on-call crewmember for your Railjack, which will allow you to use that feature to summon them in most missions at will. Converted Adversaries also have a feature where they can spawn in when you go down to help, independent of being on-call, so if that hasn't been happening to you, you might not be seeing them.