SQL Server User Defined Function Treating String As Integer

I have a User Defined Function in my SQL Server 2000 database which takes a string and adds an integer quantity to it.  The function basically takes the string (string because of first character and spaces) and adds an integer to it creating a new number (starting number and ending number concept).
If I pass in a string that has a letter for the first character, it works fine.  However, if the first character is an integer it trims out all 0s and whitespace and ruins the necessary formatting.  Note, the formatting is always the same - x xxx xxxx xx.   Any help or ideas would be appreciated.
Example:
D 499 8900 01     plus a Quantity of 10 returns D 499 89000 11  which is perfect.

However,
0 076 0000 03     plus a Quantity of 1 returns  764  764     (it should be 0 076 0000 04)
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CREATE FUNCTION  [dbo].[NEW_End_NR2](
  @OldStr as char(20), @Quantity int
 )
RETURNS @NewNR_Tbl TABLE (New_EndNr char(20) primary key)
AS 
BEGIN
 DECLARE @NewStr  char(20)
 DECLARE @addNr  integer
BEGIN
 
 Set @addNr = @Quantity
 set @NewStr = (select REPLACE(@OldStr,left(@OldStr, 1),''))
 set @NewStr = (select REPLACE(@NewStr,' ', ''))  + @addNr
 set @NewStr = (SELECT left(@OldStr, 1) +
   SPACE(1) + left(@NewStr,3) +
   SPACE(1) + left(REPLACE(@NewStr, left(@NewStr,3), ''), 4) +
   SPACE(1) + left(REPLACE(@NewStr, left(@NewStr,7), ''), 3))
 BEGIN
  INSERT INTO @NewNR_Tbl (New_EndNr) VALUES(@NewStr)
 END
 
END
    RETURN
END