With a type 2 (240V) EVSE, you can charge and preheat the cabin. With the stock charging though (level 1), set the preheat timer to run after any charging timers and realize that that doing so will use some pack battery.
To put it another way, the car always draws its climate control power from the pack. If the charge being supplied (by L-1 or a low power L-2) is less than that being used for preheating, the net charge will drop. If they are the same, there is no net loss or gain of charge. If you use a charging station that can provide the full 27.5 amps the Leaf with 6.6 kw charger can use to charge, then it will charge fairly fast while also preheating. I use L-1 to charge, so I limit preheating to 2 to 5 minutes, making sure the seat and wheel heaters are on. This gets the car from "frigid" to "just a bit too cool" and preheats the seat and steering wheel, making it much easier to get warm as you drive. The net loss is 2% to 5% of charge with L-1, for that length of time.