I was not familiar with 6-3-0 bikes, so I visited their website.
The pedal bikes seem fairly generic - with all having rim brakes. That would be my biggest concern. Every eBike that I have seen uses disc brakes, either mechanical or hydraulic. Even 6-3-0 sells eBikes, and they all have disc brakes. eBikes are heavier due to the motor and battery, and people tend to go a bit faster because they can! That's why they use disc brakes on their own eBikes.
I'm sure it could be done, and I'm sure there are eBikes somewhere with rim brakes. It's just not something I would personally do.