Funk committed to the project in February, IOS said.
Experienced supersonic pilot Chalmers "Slick" Goodlin had been appointed to the IOS board of directors.
"Both Wally's and Slick's levels of experience is unparalleled and invaluable to us. We are elated to have them on the InterOrbital Systems team," Randa Milliron said.
IOS has been developing its space craft, the Neptune Spaceliner, at a plant in the United States' Mojave Desert, where it began producing rocket engines and space launch vehicles in 1995.
The reuseable rockets being developed can be launched at sea or from a land-based space port and would be able to put two pilots and four tourists into orbit for up to a week.
Funk, a former flight instructor now in her early 60s, trained last year at the Yuri Gagarin Cosmonaut Training Centre in Star City, Russia, where she was flown at high altitude at zero gravity.
US millionaire Dennis Tito became the world's first space tourist in April last year after reportedly paying US$20 million ($45.6 million) for a seat on board a Russian rocket to the international space station.
Tonga has previously used unorthodox schemes to raise money, including the sale of passports in the late 1980s and the sale of satellite slots.
- REUTERS
Tonga - a troubled kingdom