Joel and Ethan Coen’s Bitter Creek Prods. has agreed to become signatories to Hollywood’s below-the-line union starting with their current feature, “The Big Lebowski.”The Coens, whose low-budget indie features include “Fargo” and “Blood Simple,” also have agreed to sign a special low-budget agreement for movies they make that are budgeted between $5 million and $7 million, according to the Intl. Alliance of Theatrical Stage Employees. Such a low-budget agreement, the first of which was signed by the Motion Picture Corp. of America, gives producers special breaks, bringing union wages in line with non-union indie features. “The Big Lebowski,” however, is budgeted above $10 million, and will not be part of the low-budget agreement. Instead, it will be subject to the terms and conditions of the IA’s standard contract. “Lebowski” is currently shooting in Los Angeles.