Keep/Retain Leading Zeros When Importing Text File

i cant do this from importing the data, so instead i have to try and fix it with code. I get a sheet of data from an external source that removes the 0's in the front of the number. Its only a 4 digit number so 21 would be 0021. Now i have been able to trap the data i think i would need. i created a =len() statement to tell me how many numbers each cell has. here is the code i think how it would work.. i just need a little help with filling in the blanks. I would assume its something to the lines of " 00+string value if it was 2 numbers needed.

B2 has the Len values (4,3,2,1 etc) and A1 is the original string that has the values.
So i want the code to add 1 2 or 3 zeros to the front of the number based on the len value.

Range("B2").Select
While ActiveCell.Value <> ""
ActiveCell.Offset(0, 0).Select
Select Case ActiveCell.Value
Case Is = "3"
ActiveCell.Offset(-1, 0).Select
ActiveCell.Offset(1, 1).Select
Case Is = "2"
ActiveCell.Offset(-1, 0).Select
ActiveCell.Offset(1, 1).Select......................