Boko has been capped by his country at sevens and fifteen-a-side level, most recently on the Pacific Islanders European tour in 2009.
"I had the pleasure of working with Asaeli on the European tour last autumn and was impressed with him," said head coach Mike Brewer.
"He has been with us now training and conditioning and he has earned his contract to play for us."
The Manchester-based outfit have begun their European campaign in emphatic style, scoring 153 points and 22 tries in their first two matches.
When European rugby returns in December they face back-to-back fixtures against Brive who are also unbeaten in Pool 2.