You can not (as of now) since they are compiled (Pandemic's msh format). Nilot is working on converting msh to MilkShape 3D ascii format which you should be able to convert to XSI format.
Now, for other way around, make your model in XSI (or what ever you use), import it to MilkShape, export it as Milkshape 3D ASCII and use Nilot tool to convert it to Pandemic msh.
The tool (BConstuctor) is not out yet, but it should be soon. This is more/less how you will be able to convert models back and forth.
Now, if XSI plug-in comes out this will be much easier.
Follow progress on BConstructor at: