Can the extra players extension work with Seafearers or Cities and Knights?
Seafarers and Cities &amp; Knights have their own 5-6 Player expansions in addition to the base game 5-6 player expansion. Think of them as expansions to expansions. So to play a 6 player game of Seafarers you will need the following: 

Base Catan Game + 5-6 Person Base Expansion + 4 player Seafarers Expansion + 5-6 Person Seafarers Expansion