Some sedevacantists believe that an antipope can validly appoint bishops under supplied jurisdiction for good of the Chuch - that is an attempt to reconcile sedevacantism with the problem of Apostolic Succession.
It was an interesting thought, suggested by none less than John Lane, but it doesn't hold up under some extremely important considerations.
However, there have been, at times, antipopes who have made episcopal appointments which were allowed to stand and be accepted as having been lawfully appointed. Whenever this happened however, the antipope involved was nevertheless still a Catholic, someone who could be followed by Catholics without risk of their soul (in terms of his teachings and rulings and other official actions). In fact, very few of the 41 big historic antipopes have ever been heretical or even erroneous: Novatius is one (who went on to start the Novatians) and Vigilius is the other (who favored the erroneous "three chapters" and the three clerics who pushed them while Pope Silverius was still alive, but who turned against them when he became Pope), being the only two I know of.
But these were all antipopes who attempted a Papal reign while some other legitimate Pope also reigned. Paul VI and those coming after him have sometimes been called "antipopes," but their papal delusions are rejected not owing to the presence of a real Pope (unverifiable Siri hypothesis notwithstanding), but owing to their flagrantly heretical contradiction of the infallible teachings of the Church as confirmed by 260+ real Popes of the Church; they are in fact more properly referred to as heresiarchs.
A true Catholic, mistaken for a Pope by himself and by some part of the Church, would do his best to choose truly qualified Catholic men to be bishops, if anything competing with their papal claimant rivals for sheer orthodoxy in their choices and actions. A true Pope could therefore reasonably confirm and approve most or even all of the episcopal choices and appointments by such an antipope.
But a heresiarch appoints fellow heretics of like distorted mind and distorted theology, and as such none of his episcopal appointments should ever be permitted to be confirmed or approved. What sheer insanity it would be to assign to a heresiarch the authority to decide who is to be "approved" as a Catholic bishop and who is not! The John Lane suggestion, reasonable enough for most historical antipopes, can have no meaningful application to our present day situation wherein what we have is no mere antipope but heresiarch.