https://www.airmodsflightcenter.com/ferry-tank-installations
At the end of the article is a description of adding ferry tanks to an ATR 42.
excerpt:
In January 2011, a major commercial carrier operating out of an island in the Western Pacific Ocean contracted Air-Mods to install the Turtle-Pac ferry tank in an ATR-42. The purpose was to allow the airplane to fly the long trans-Pacific legs between the Western Pacific Ocean, Hawaii, and California so the airplane could undergo scheduled service here in the mainland U.S. Air-Mods engineered the installation to comply with the Federal Aviation Regulations and the ATR design requirements.
One of the challenges related to the fuel line that had to pass from inside the cabin out to the wing fuel tank. Since there were no available openings from the cabin to the wing root, Air-Mods engineered a panel that would temporarily replace a cabin window to accommodate the fuel line. The challenge was to do this without disrupting the fuselage aerodynamics and also to maintain the envelope pressurization.