Yoav,
you are a lucky person as the internal strobe of my Ikelite D90 housing don't open enough that the internal strobe will fire.
This excludes the option of using optical triggered slave strobes.... 
You may buy a slave strobe, optical cable and a mounting block for the camera to trigger the slave strobe,
as it is normal on compact cameras. To avoid that the light of the internal strobe will disturb use either:
1° masking tape to cover the area in front of the internal strobe, leaving only a pinhole where the optical cable mounting block sit
2° cover the internal strobe with a piece of black fully exposed 35mm film as this will still trigger the strobe, but no visible light escape the housing.
I am not sure but pretty sure that the INON D-2000 strobe will handle the strobe from the internal strobe and provide you pseudo TTL using a
optically triggered slave strobe. Other brands of slave strobes may work also, a try will answer this question to you.
Chris