The only previous camera I'd used with DX coding (Canon Top Shot way back) worked anyway but treated all films that it couldn't read as ISO 100. The way I saw it this wouldn't be a problem as long as the film is ISO 100. Apparently they have some kind of safe guard on the Xpan that does not let you use film with unknown sensitivity (set it through the DX code or manually and the Xpan is happy). In all my other cameras, including Nikon F4 I set the speed manually so I have never really thought about the possibility of this kind of safe guard or whether Silvermax has a DX code or not. My question was quickly answered thanks to apug and I can start using Silvermax in my Xpan.
About spooling. The Xpan, like some other Fuji cameras, starts by spooling all the film onto the take up spool and the proceeds to wind it back into the cassette when you take pictures. This is done to make sure that you don't loose any pictures should you accidentally open the camera. The safe guard disables this initial spooling until the ISO is set either by auto or manually. I just have to remember to set the speed manually for it to spool.