well, you can buy the individual Mahle components from Orbea, but you will run into a problem with the range extender. It’s not a simple battery, but a battery charger, that trickle charges the main battery so it can’t be used alone. Also, the system controller is built into the main battery housing, so you will still need that. In any case, the system is possibly underpowered where you need it most, on steeper climbs, as the motor is most efficient nearing its cut-out speed of 25kmh (in Europe).
I’m afraid you will need to be looking at a conversion kit of some sort, but you can certainly hide a battery in a saddle/seat post bag for stealth. I have a 7Ah hoverboard battery in mine, very small. You will need to hide the controller, but gravel bikes look cool with frame bags, so that would work. Maybe ask on here for recommendations on conversion kits. I used a Yosepower one for a year, it was amazing, actually better than my Mahle powered bike, but unsure of their longevity.