Add a single quote/apostrophe in front of the number (without a closing quote at the end), so that it reads it as text and does not auto-format it as number.
For example you have:
12345.
Excel will auto-format that column as a number and drop the period unless you specify that it's a text only column.
Change it to:
'12345.
Excel will read that value as a text column and will not save the apostrophe in a text-based CSV file.
Edit: In testing this, I found that ff you save it as "12345.", Excel changes the CSV file to """12345.""".