Many pathology fellowships allow this if you are board certified by another specialty. But usually the time of the fellowship is extended to 2 years or longer. Residents who complete pathology (AP, AP/CP/or CP) will only complete one year of fellowship because we have background knowledge from our primary residency. Here is the site which will detail requirements for each subspecialty fellowship. The only pathology fellowships exclusive to pathology are cytology, forensic pathology, pediatric pathology, and molecular genetic pathology. If you read the requirements, there is usually a described alternate pathway for those who are certified in another specialty.
American Board of Pathology