To do it, use below user mapping script in advanced setting of repository definition:
groovy:
import org.hibernate.criterion.Restrictions;
import com.pmease.quickbuild.SearchCriteria;
import com.pmease.quickbuild.entitymanager.UserManager;
def criteria = SearchCriteria.of(Restrictions.ilike("name", committer));
def users = UserManager.instance.search(criteria, 0, 1);
if (!users.isEmpty())
return users[0].name;
else
return null;