Redknapp said: "Jermain is a natural goalscorer who will get 18-20 a season. He's done it again for us.
"This will be the second Wembley match he has missed but, yes, of course we will be taking Jermain to Wembley with us - hopefully twice."
Defoe scored twice in the first nine minutes, including a penalty, as hosts Pompey looked to be continuing the celebrations following their stunning success over United in the cup quarter-finals.
But Birmingham pegged them back and were level by half-time thanks to strikes from Fabrice Muamba and Sebastian Larsson.
It took a Hermann Hreidarsson effort to get Pompey back on track and Kanu wrapped up the win late on.
Centre-back Sylvain Distin was caught out for Birmingham's first goal and Redknapp said: "He's been with his wife all day at the hospital where she's expecting and it looked as though his concentration suffered in the game. I just hope the baby is not born on semi-final day.
"But it was nice to get four goals at home because we haven't got many here since the seven against Reading.
"I'm told we're seventh in the table now and it's going to be a tough scrap for the European places.
"Credit to Birmingham for coming back from two down because it looked like four, five or six we could
get at that point. "It was a good game to win and the atmosphere out there was great."