I'll let the Legal Eagle's help you figure out what needs to be done with the FAA.
I flew my -10 for 100 hours, give or take, before deciding that 60 gallons was not enough for a true IFR mission. Alternate + Reserve fuel can really eat into stock tank range. I added 15 gallons a side, one bay outboard of the main tanks. They automatically transferred into the mains, keeping the Primary fuel system stock. I then emailed the local FSDO, put the airplane back into Phase 1 and did a 5 hour flyoff. Probably one of the best mods I ever did as far as aircraft usability.
Now, your -14. Couple of ways you can add fuel, easily.
1. Hotel Whiskey ER tanks. They will give you 6 gallons a side. Quite popular, keeps the fuel in the wings, overall probably the best option.
2. Use Smoke System Helper RV-14 behind the seat tanks. 4.8 gallons a tank, two will fit. Downsides, only 9.5 gallons, fuel is in the cockpit (really not a fan), if you spill when filling up.. well, you already know.
3. Cut the leading edge or a section of it off and replace with additional tanks. Not for the faint of heart. However, the capacity can be whatever you want it to be, only limited by the wingspan.
As I said, I added 15 a side on my -10 bringing total up to 90 gallons. For a 540 @ 14gph it was perfect for 3.5 hours + Alternate + Reserve. I have seen -10's with up to 120 gallons, but I think that was for simplicity of tank construction. I'm fairly sure they just built another set of main tanks and mounted them up.
As Mark attempted to point out, Aerobatics with fuel in the Aux tanks (wing) is probably not a wise move. You would think that it goes without saying, but I guess not! ;-)
Nick "Rehab" Nafsinger
91TS Rosharon, TX
RV-4 Bought 2019, Day to Day Restoration while still flying. SDS EFI IO-360, Hartzell CS, DanH inspired custom intake, G3x Panel
RV-8 SB, "Back burner project."
RV-10 Built 2010, Sold 2019