XL's stored precision is limited to 15 decimal digits. If you're not
doing math on them, pre-formatting the cell(s) as Text or prepending the
numbers with an apostrophe when you enter them will keep XL from parsing
them and dropping the last digits.
If you need to do math on them, you'd best use a different application.
Otherwise you'll have to enter them as Text and use string manipulation
to do the math piece-wise.