Yes, it has been done before.
I just jad this discussion with another member of this forum last night.
Here is what we discussed:
1. Decide how much extra fuel you want. Then divide 18 by the number of bays in the tank and cut it down to meet your requirements.
2. Make the entire piece of leading edge that will bridge the two tanks removable rather than just adding an access plate.
3. Do the RV-10/Rocket coil for the vent line outboard of the tank and poke it out the bottom of the wing.
4. Getting the fuel to the engine is a bigger debate than the nay sayers will have about adding these tanks. You can add electric fuel pumps to move the outboard fuel to the onboard tanks. My favorite idea is to buy a valve (Andair?) that has five positions (Left outboard, left inboard, off, right onboard, and right outboard). That is the simplest to plumb and doesn't require the complexity of fuel pumps, wiring, fueses, etc.
Good luck and please report back with your configuration.