The Cranes winger was top scorer at the 2010 regional Cecafa Championship with four goals scored in five games
Okwi plays for Simba in Tanzania and will arrive at Mayfair in a deal worth a reported R7-million (€680,000), a record transfer in Tanzanian football.
He had a trial at Kaizer Chiefs before last season and was asked back for a second viewing, but at the time turned down the chance to instead attend a another trial in Turkey which came to nothing.
The 19-year-old already has 11 internationals goals to his name for The Cranes, having made his debut at 17, including netting in the 1-1 draw with Angola in a 2014 World Cup qualifier against Angola on June 2, and in the 4-0 win over Congo in the 2013 Africa Cup of Nations qualifier this past weekend.
He was also top-scorer in the 2010 regional Cecafa Championship with four goals in five games, and matched that number in the 2011 tournament too.
"The deal is already done, but there are only a few things that the two clubs have to settle before giving Okwi the green light,” Simba communications officer Ezekiel Kamwaga told reporters.
"We have lost one of the best players who helped Simba win the title last season; it will be difficult to plug the hole left by him, but we are happy for it was a lucrative deal.”