Why? It is far more easier to just login into Flux and a msg box will pop up and say "Fluxbox is not your default Desktop, would you like to make it?" and it will give you two options, "Just for this session" or "keep as default"
(Iím not to sure on the actual wording but its something like that.)
The only other way I know of is too edit the desktop file, Iím not on Linux at the moment but I think it is /etc/desktop (I'll confirm this when I get home) and you can change the settings there and save it. (Will need to be root)