Upvote:2
I'm afraid you're out of luck. The information page about WIZZ Flex does not list any other way of purchasing it apart from when making a booking. Given the nature of WIZZ Flex, it doesn't make much sense to allow adding it later. If that was possible, anyone could avoid paying cancellation/change fees or the Wizz Flex fee by only buying WIZZ Flex when they actually needed to cancel/change.
You can try calling WIZZ Air and asking them if they can add WIZZ Flex to your booking, but as I said, your chance of success is likely minimal.