You might have the answer to this now, but I've been looking into this myself (considering purchasing a Strive On), yes it would appear to be compatible:
Give your tour more power: With the PowerMore 250 Range Extender (additional battery), which is attached to the mounting points on the down tube, you can extend your ride by approx. 60%.
If you click on "compatible products" in the link above it says it's only compatible with the Grizl:ON.
Someone has tried to fit one in the facebook Strive ON group and there isn't quite enough room.