Movie critics Elvis Mitchell and Joel Siegel will soon give their thumbs a workout alongside Roger Ebert as guest reviewers for the syndicated series “Siskel & Ebert.” Mitchell, film critic for the Ft. Worth Star-Telegram and entertainment critic for National Public Radio’s “Weekend Edition,” will be featured on next weekend’s program. No airdate for “Good Morning America” critic Siegel has been set. Roger Ebert, surviving partner of “Siskel & Ebert” (Gene Siskel died two weeks ago) has said he intends to continue the popular movie roundup series, but the show will retain the “Siskel & Ebert” name through the end of this season in tribute to Gene.