Board Thread:News and Announcements/@comment-24264457-20180226215326

One month from now, on March 26, 2018, the ability to add Wii Points to the Wii Shop will be removed in preparation for the discontinuation of the Wii Shop on January 30, 2019. So if you are out of Wii Points and you still want to buy some DLCs from a Wii JD game, you have one month left to do so.

So to recap, you can add Wii Points until March 26, 2018, and have until January 30, 2019, to spend them.

More info can be found here. 