I do mostly agree with Michael; however, it is possible to deviate from standard E-6 formulas. You'll probably get something unsatisfactory, or at least weird. Cross-processing of E-6 film in C-41 chemistry is an example, but you could experiment with the first developer formula in an otherwise standard E-6 process to get positive images in some other way. You shouldn't expect good results, though, at least not if what I've heard is correct; changes that are likely to affect features you're interested in affecting (grain, acutance, etc.) are likely to have undesirable side effects.