There are various theories about architectures that use a Dragon or Starliner to transfer astronauts to the respective HLS vehicle in low Earth orbit before then departing for the Moon.

There doesn't appear to be too much of a reason that shouldn't be possible, particularly since both HLS vehicles are meant to be able to perform orbital refueling, but the idea has oddly remained absent from official public documents.

My personal theory is that like with orbital fuel depots, the idea is being kept out of official public communications until the HLS vehicles are operational (and thus uncancellable), at which point the political will can be more easily gained to cancel SLS without risking another waste-of-life threatening to kill the entire human spaceflight program if it didn't directly buy him votes.