Only players on your online roster that signed the waiver (during the first 3 weeks) are eligible for the post season. Any players that are new, not on the roster, or joined mid-season are not allowed in the playoffs.
If your team is in danger of forfeiting a playoff game, you can request a substitute here: https://www.underdogseattle.com/page/Playoff-Substitute
Playoff subs must be approved by the office ahead of your game, and cannot be approved by a staff member at the fields. If approved, the player will be added to your online roster.