The 18-year-old, who spent last season on loan with Monaco B, has attracted interest from other Premier League clubs including Manchester United and Arsenal, but Martinez says a deal is on the verge of completion.
Henen has scored at four different levels of Belgium's national youth side, and the Toffees boss said the move, thought to be for a fee of around £1.5m, was "in the process" of being finished off.
"He's a young, talented footballer who represents Belgium in the lower teams, in the Under-18s and has had good experience in going to France," said Martinez after Everton's 2-2 draw at Tranmere Rovers in a pre-season friendly on Tuesday.
"He is a bright boy and we feel he is something different that we haven't got in the Under-21s, so we are hoping he is going to be giving us something different in that group.
"We want a strong group. We have already got talented and very interesting footballers and I think David can fit in really well in that environment."
The draw with Tranmere saw Seamus Coleman withdrawn after 45 minutes with an injury, but Martinez was not too concerned by the defender's fitness.
"Seamus came off as a precaution because he felt his hamstring slightly. Apart from that, it's been a real success in what we wanted to achieve."