If you want your number to remain a numeric value, you'll need to use cell
format: press Ctrl+1, select the Number Tab, choose CUSTOM from the
categories list and enter the following in the box called Type: 000000#
If it is OK for your number to become text, then use the TEXT() function.
E.g. if cell A1 contains 12345 the formula =TEXT(A1,"000000#") will return a
text string which will look like this 0012345. To convert it back to number
you can use the VALUE() function: =VALUE("0012345") will return 12345 as a
numeric value.
Hope this helps,
KL