Former Supreme Court nominee Robert H. Bork may leave the U.S. Court of Appeals for the District of Columbia to join the conservative think tank American Enterprise Institute, according to a published report. Newsweek, in its latest issue, quoted sources close to Bork as saying he is preparing to give up his court seat, possibly by the end of the year, and is involved in discussions with the conservative organization. The magazine said that Bork has told friends that he is bored with the administrative law cases that dominate the court docket. Bork declined comment on the report. In July, President Reagan nominated Bork to succeed retiring Justice Lewis F. Powell Jr. Four months later, the Senate rejected the nomination.