World Series of Fighting has bolstered its heavyweight ranks with the signing of Derrick Mehmen . Sources close to the fighter confirmed his signing to MMAjunkie.com (www.mmajunkie.com), saying Mehmen (15-5) has inked a multi-fight deal with the Las Vegas-based promotion. Mehmen will move up from light heavyweight to fight at heavyweight for WSOF. His debut with the promotion likely will come later this year. “Caveman” Mehmen has a pair of Strikeforce fights on his resume, going 1-1 with a decision win over Roy Jones on a 2011 “Challengers” show before dropping a decision to Gian Villante 13 months ago