Mother born Thai and now has U.S. Citizenship children have accompanied her and are now U.S. Citizens as well. Children have resided outside Thailand since 2002 living in Korea, Japan and now U.S. The children still have Thai passport as well. I'm dad to them.

Stepfather desires to adoption children. I'm assuming Thai father must have given parental child rights to mother during divorce, because we've had no problems with visa for Japan and during the whole immigration process. To ensure a smooth and timely adoption the state of Pennsylvania needs consent of biological father. Do I need to track him down and have him sign additional paperwork? Children do not know who there dad is and have not had any contact since I came into their lives in 2002.

Any suggestions/recommendations?

As you state yourself, it would make things easier. So yes, track him down.

(Thai law says he must give permission, but if he refuses a Thai judge can give permission instead to go ahead with the adoption. But that would also require to first locate the father).