I'm not sure that I understand what you mean by "rights of ownership".
If User B owns a stored procedure, User B can grant User A permission to run the stored procedure
GRANT EXECUTE ON b.procedure_name TO a
User A would then call the procedure using the fully qualified name, i.e.
BEGIN
b.procedure_name( <<list of parameters>> );
END;
Alternately, User A can create a synonym in order to avoid having to use the fully qualified procedure name.
CREATE SYNONYM procedure_name FOR b.procedure_name;
BEGIN
procedure_name( <<list of parameters>> );
END;