Ma'afu, 33, has appeared in over 50 Tests for Fiji and has experience of playing in Britain after spells with Cardiff Blues and Northampton Saints, while 27-year-old Feao featured in three Tests for Tonga in June.
The duo give Leicester greater depth in the front row after Genge sustained a knee injury while on international duty with England.
"We're obviously disappointed to have Ellis sidelined for the start of the new season after suffering a knee injury on the England tour … and we look forward to seeing him back to full fitness in due course, but Campese is a very experienced prop with good knowledge of Premiership and European rugby," head coach Matt O'Connor told the Tigers' official website.
"David comes in with some good experience in Europe and we think he can continue to make good progress here. He is a big, powerful man and we look forward to getting him to work in the build-up to the new season."
WELCOME TO THE #TIGERSFAMILY ... CAMPESE MA'AFU!
Leicester Tigers have added Fiji international prop Campese Ma’afu to the senior squad ahead of the new season.
— Leicester Tigers (@LeicesterTigers) August 7, 2018
Photo Getty Images. Caption: Rugby player Campese Ma'afu