A genealogist from Nova Scotia who takes live phone-in calls once a month on CBC Radio in Halifax was asked about the name BUCKMAN on the July show phone-in last week.
He said that BUCKMAN and BUCKNAM could be different forms of the same surname, and that the name might have come from BUCKENHAM or BUCKINGHAM in the Norfolk-East Anglia area of England, north of London (the port of Great Yarmouth is nearby).
As you likely know, "-ham" on the end of an English word meant "settlement" or maybe "village". "bucken" sounds like it might have German or Dutch roots from long ago, but that is only my thought. My feeling is that it is not connected to the Normans and William the Conqueror of 1066. I think these people were already living in England when the invaders landed.