These are examples of the formula needed to zero-fill a numeric field.

Example 1:
Length = 10 characters
Convert = 1234 to 0000123400

Right(("0000000000" + toText(({NUMBERFIELD.EN_US}*100),0,"")),10)

Example 2:
Length = 10 characters
Convert = 1234 to 0000001234

Right(("0000000000" + toText({NUMBERFIELD.EN_US},0,"")),10)