It looks like the best thing you can do is to assume that unpolarized => equally polarized in the x and y (or whatever coordinates you are using) direction. When you say light is unpolarized you are saying that it is randomly distributed. You can always break this down into polarization in x or y direction (assuming the light is traveling in the z direction), and if the light is truly random, there should be roughly 50% of it polarized in the x direction and 50% polarized in the y.
"Note that Jones calculus is only applicable to light that is already fully polarized. Light which is unpolarized, partially polarized, or incoherent must be treated using Mueller calculus."
Guess you can't, it says you must use mueller calculus.
I think Muller calculus can be used to modle polarized and unpolarized light, but it is easyer to use Jones calculus is easyer to used on fully polarized lightthan Mueller calculus.