It's something I've been wanting for my Sims homes forever! There's some houses where I simply can't imagine the fridge without some bills to pay, kids drawings, or a recipe pinned on the door.
Buuuuuuut Sims 3 fridges refuse to have slots!!
So, I had to create what I called a slot panel (required), that needs to be placed in front of the fridge, and then that can be invisible, just like OMSP.
(See pictures below)

Other not completely satisfying point: decors won't move with the door (even if they were effectively on the door), and it annoys me to see the food in the fridge, just behind the decors. I liked better removing the door animations, so the door doesn't open, Sims can still use them though. I've converted 8 fridges from Sims 4, all unanimated.

Now, for the good news!
You can use all my Pinboard clutter for the fridges!
You can put your own Sims photographes, thanks to a frame that you can put on the fridge.