When you say, "Fired by the on-camera flash" you are defining the use of a "slave", plugged into the flash unit and fired by the on-camera flash, with *no* hard connection - therefore synch circuit current/ voltage does not come into play. That should be, simply, *no* problem ... one can use a variety of accessory "Slave triggers", and plug them into anything, from a very small battery-operated flash unit to a VERY large Studio unit.

The question would lie with the in-camera flash system itself... would the firing of an external flash synchronize properly? Would the in-camera flash control system/ exposure system get all "confused"? If it is possible to set exposure and the flash itself on "Manual" - I would expect no problem - from there...

The "Peanut"-sized slave triggers are relatively inexpensive ... I'd buy one and give it a try with a borrowed - existing ... flash unit before I became too involved.