Can you adjust your status from H2B to Immigrant?

The problem with this scenario is that an H-2B must show they intend to return home at the end of their H-2B status and with a permanent resident or immigrant visa application you are indicating an intent to return. There are many different types of permanent residency processes to immigrate. Some can be completed very quickly and others take years to complete. Some require a showing of maintenance of status and some do not in order to adjust. Best would be to know more facts regarding what type of immigrant visa you are thinking about. If it is one of the shorter types of PR, then it may be possible to adjust while still in the U.S.