I tried playing around with the mod editor when I tried to make the pacific Gavutu map into an assault map. Well, it appears that no one in the community has yet to ask this question (i searched).
Assault game mode works similar Invasion. Sandbox Portal page with official guides.
Just need to use chained capture points and in properties panel enable capzone__checkAllZonesInGroup option (for group points you need set similar groupName and capzone__activateChoice).