Industry performers and advertising agencies have agreed to extend their contract until April 18 after nego-tiations failed to produce a new agreement before the pact expired.Both sides are trying to come up with a new contract covering work in commercials. Reps from the Screen Actors Guild and the American Federation of Television & Radio Artists recessed their talks Thursday. Had the pact not been extended, the guilds would have been left without an agreement after it expired today. The talks are scheduled to resume April 14. According to a recent study, commercial work represented almost a 34% share, or $516 million, of the reported earnings among SAG members last year; a large portion of that income was from residuals. The negotiations are being led on the agency side by John A. McGuinn, chief negotiator for the joint policy com-mittee of the Assn. of National Advertisers and the American Assn. of Advertising Agencies. Ken Orsatti, national executive director or SAG, and Bruce York, national executive director of AFTRA, are repping the guilds.