Parameter values:

cf2 = 1137
di2 = 2011-01-01
cf1 = 1137
di1 = 2011-01-01
df1 = 2011-12-31

Query:

SELECT * FROM
(
    SELECT
      CAST(NULL AS DATE) AS DataMov, 'Saldo Anterior' AS Descricao, '' AS TipoDoc, CAST(NULL AS INTEGER) AS NumReg, '' AS NumDoc, 
      SUM(ValorMovCS) AS Valor , CAST(NULL AS INTEGER) AS Diario, CAST(NULL AS INTEGER) AS NumLanc
    FROM
      MovFor
    WHERE
      CodFornecedor = :cf2 AND DataMov < :di2

    UNION ALL

    SELECT
      DataMov, Descricao, TipoDoc, NumReg, NumDoc, ValorMovCS, Diario, NumLanc
    FROM
      MovFor
    WHERE
      CodFornecedor = :cf1 AND 
      DataMov >= :di1 AND 
      DataMov <= :df1
)
AS TmpTbl