External Flash with Z990?
I've used both of these accessories with a Vivtar 285
Wein PN Peanut Slave (100' Range)
The sunpak is discontinued.. but this Morris (below)
looks to be a reasonable substitute..?
Morris DS-1 Digital Slave Trigger with Camera Flash Bracket
Otherwise, Metz makes an external flash, that mounts into
the tripod hole.. but it's pricey.
With all these solutions, they will work fine, until you are in
a room with a bunch of other flashes going off..!