Number as Text to Date
One way to do what you want is to format the column your entering the six digit date into as a text field (say column 'A' ). You can then convert the text number in a seperate column (column B) to date format. If cell A1 has '021308', the the formula in B1 would be =LEFT(A1,2)&"/"&MID(A1,3,2)&"/"&RIGHT(A1,2) will convert the text number to 02/13/08.
If you need the cell with the formula converted to a true date field, then in a third column formated for dates you can select the column with the text numbers in column B, right click and select 'Copy', move to the top of the column formated for dates, right click and select 'Paste Special' then choose 'Values' on the Radio Button and click 'Okay'.
You then have a column of text numbers formated for dates. You can then hide or delete the columns with your original date entry and the calculation column.