Yes, you can add days to a MYW ticket any time before it expires (14 days from first use)
ADDING DAYS TO MYW TICKETS
MYW tickets can have days added to the ticket. If you have an expiring ticket, you must add the days within the 14 day from first use window. No ticket can exceed a total length of 10 days. If you have a 10 day and used 5 days from it, you cannot add another day to it as the total cannot exceed 10. But if you have say a 7 day ticket and you use 2 days, you can add on up to three additional days.
There is no limit to the number of times you can upgrade the ticket (as in adding only one day at a time each time you upgrade) but in all cases, the total admissions cannot exceed 10. Park Hopper and Water Park Fun & More add ons do not count in the total.
Depending on where and when the upgrade is performed, a new ticket may or may not be issued. Typically, a new ticket is issued if it is upgraded in a system different from its original issuance media (i.e., resorts stock, paper stock at the parks, etc). Regardless if a new ticket is issued, the original biometrics information will follow the ticket through upgrade.