How to occupy lvl 5 territory?

Level 5 plots are found in the outer most region of the state map and also the next region in. You need to have an adjacent plot to it to occupy it (either your own or an alliance member).

Upgrade your virus research buildings to get your immunity high enough to occupy ti successfully and your APC will need to be strong enough. The game will warn you before attacking if your APC is not strong enough.