Decreasing Frequency

Is it possible to run a 4-day program over 3 days (4th day will run over to next week)? Or even spread out a 3-day a week workout plan into 4 days with the 4th day rolling over to next week? My understanding is that this reduction in frequency has to be met with a raising of volume.

Why would I want to do something like this? To have shorter workouts and/or to incorporate more accessory work (biceps, triceps, chins, etc…)

I wouldn’t make those modifications for the reasons you listed, but rather would reduce rest periods, use things like myoreps or emoms, and try to complete the workouts faster.