Throw it at them like you would toss an item out of your inventory, ensuring it does not get thrown out of the pen. You can hold Shift (or control?) when you click on the item in your inventory and it will get automatically tossed in the direction your character is facing.

Helpful tips:
1) While you are out foraging/exploring, grab all the mushrooms and raspberries you run across.
2) If you find a gathering of each, mark it on your map and everytime you run by there, grab them.
3) Craft a chest and put it in the pen with your boars. Put all your extra shrooms, berries, carrots etc in there when you return from foraging. This way you will have a supply of boar food right there in the pen and won't have to go into your house and back again to feed them when they run out.
4) You could also store the meat, pelts, and trophies in the chest untill you get a bunch and get around to putting them into your main storage containers.