If you trade for a UFA rights after playoffs can you sign them to a 8 year contract? I saw something a while back that you can't but wanted to make sure

They have to be on your reserve list as of the most recent trade deadline in order to qualify for the 8th year.
So the answer to your question is that you can't get the 8th year by acquiring a player's rights, but you can if it's done through a sign and trade, where the original team signs the 8 year contract prior to the trade.