I'm assuming you mean this vehicle:
It, like all other Hybrids probably uses an undersized electric motor and controller.
This means you'd either have to add a second motor and controller system.
Or replace the motor/controller inside the Highlander for higher power ones that can handle highway driving.
The batteries included in the Highlander are probably not usable because you'd have to either completely disassemble the battery pack or rebuild it into a bigger battery pack or toss it and build a new pack.
The batteries are also most likely NiMH cells.
It would probably be more costly to do this than to start with a cheap used ICE version and convert that in an ideal way.