So what do you need?
- a proprietary flash that can do HSS/FP
- A flash with manual settings
- An optical slave
- Optional: Radio trigger and a way to attach the optical slave to radio trigger
If you are using a Nikon camera with PC port I believe you can do this without the optical slave and connect either the flash or a radio trigger directly to your cameras PC port.
Ok now to put it together . . .
- Put the proprietary flash on the camera and select HSS/FP mode (this is the trigger so reduce power and point away from subject if you don't want to use as a light source)
- Select full power on the manual flash
- Put the optical slave where the on camera flash will trigger it
- Then the manual flash can be connected by a number of different methods, either directly to the slave, with a PC cord or a radio trigger can fill this gap, i.e. the transmitter can be attached to the optical slave and the receiver to the flash.
And . . . Ta Daaaa, you have a system that should give you HSS using a camera with a mechanical or focal plane shutter. You still need to remember that we are now using the flash as a continuous light source rather than an instantaneous flash, therefore shutter speed will now affect the exposure of the flash. Because the "continuous light source is not on for long and varies in power you will also see some light drop off across the frame, particular as the shutter speed gets higher. This also means that you will not get the large relative "gain" in power you can when you have a camera with an electronic shutter but it is another technique to keep in the back of your mind that might be useful one day.