As long as at least one reward is enabled for Late Pledges when your campaign ends, you can continue to add further Late Pledge rewards for new backers.
However, as with when your campaign was live, if a backer has already pledged to a reward you will no longer be able to make any changes to it, except for specifying new countries that you'd like to offer shipping to, adding or editing a reward image, to increase the quantity, or to change the availability. To do so, select “Edit project” from the creator tools menu, located on the left side of your project page, to edit your rewards.
Keep in mind that backers of Late Pledges are unable to modify or cancel their pledge. If you need to change a reward that has already been pledged to, you can choose to end it but any changes will need to be communicated with the backers.