Nikon D800 + SB-910 + SB-80DX
I was shooting D100 and with SB-DX as on-camera flash for almost a decade. I finally upgraded to a D800 last year, and just last month I got myself a SB-910.
Needless to say, I've only started learning about wireless flash after getting the SB-910. To which I was wondering whether it's possible to use my SB-80DX with my D800 by attaching a SU-4 Wireless Remote TTL Flash Controller to the SB-80DX (to emulate the slave setup that's built into the SB-910)?
Would that work?
Yes, but ... (two buts)
The SB-80DX already has the SU-4 slave built into it (called wireless slave). So that's all you need, you already have it.
And with the D800, the SB-80DX can only work in a Manual flash mode system. Which means its wireless remote Manual mode, NOT the Auto mode. SB-80DX manual, page 85.
The SB-80DX manual discusses it working with TTL, but which means Film TTL - it does not mean digital iTTL with the D800. It cannot do iTTL. So to use the SB-80DX in any capacity with the D800, every flash in the system has to be in Manual flash mode (and specifically, NO COMMANDER). But then, no issues.