Cook and stir chicken in large nonstick skillet on medium heat 8 to 10 min. or until done. Remove from skillet; cover to keep warm.
Add water to skillet; bring to boil, stirring occasionally to scrape browned bits from bottom of skillet. Add pasta; stir. Cover; simmer 10 min. Stir in barbecue sauce and 2 Tbsp. onions; cook, covered, 2 min.
Return chicken to skillet along with the tomatoes; stir. Top with VELVEETA; cook, covered, 2 min. or until VELVEETA begins to melt. Remove from heat. Sprinkle with remaining onions.