Basically if you cross-process slide film in C-41, what you would get is a negative image without the brown tint. The tonal curve would also be quite different. If you cross-process negative film with E6, then you're going to get a positive image due to the reversal process but with the brown tinge. Usually, most would cross process slide film with C-41 rather than the other way round, so in Photoshop you can just try inverting the colours and then tweaking the colour balance and the gamma to get a similar effect.
