Each of the presidential candidates have pledged that if elected, he will appoint a member of his transition team to work with U.S. nuclear arms control negotiators, senators who chair an oversight group said Thursday. The promises by Vice President George Bush and Gov. Michael Dukakis were made in response to requests from the Senate arms control observer group. Sen. Claiborne Pell, D-R.I., co-chairman of the group, said the pledges will help the ongoing U.S.-Soviet effort to work out a treaty making deep reductions in the superpowers' stockpile of long-range atomic weapons.