You have to go through dynamic SQL. To do that in a UDF you need a
helper procedure.
Here is a recipe (untested):

--#SET TERMINATOR @
CREATE PROCEDURE poo(IN a INT, OUT aplus1 INT)
BEGIN
  DECLARE txt VARCHAR(200);
  DECLARE stmt STATEMENT;
  DECLARE cur CURSOR FOR stmt;
  SET aplus1 = a;
  IF a < 60 THEN
    SET txt = 'SELECT FOO('|| CHAR(a) || ') + 1 FROM SYSIBM.SYSDUMMY1';
    PREPARE stmt FROM txt;
    OPEN cur;
    FETCH cur INTO aplus1;
    CLOSE cur1;
  END IF;
END
@

CREATE FUNCTION FOO(a INT) RETURNS INT
BEGIN ATOMIC
  DECLARE aplus1 INT;
  CALL POO(a, aplus1);
  RETURN aplus1;
END
@
--#SET TERMINATOR ;

VALUES FOO(1);

Note that the permitted level of recursion is 64 in DB2 9 (up from 16 in
DB2 V8)

Cheers
Serge

Perhaps if you post your actual problem, we can find a way to avoid
recursion.