Your numbers are probably stores as numbers, if you still want them stored as numbers you could use custom number formatting.
Select Format -> Cell -> Number -> Custom
and enter this format: 00000000000
this makes your number display with 11 digits and leading zeros.
If you want your numbers stored as text. With your numbers in column A, but this inn B1 and copy down:
the latest formula even works for number with less or more than 10 digits after the zero, and is probably your best choice.
You can the use copy and paste special values to convert eliminate the formulas, and then copy column B to column A, to replace the original number.