Yes they can. And they probably will. Just because it's morally repugnant doesn't mean it's implausible, I'm afraid.
Walker, on the other hand, will most likely not die as President-for-Life as old age. But a more successful Walker will have big butterflies on Central America and U.S. politics. If he survives long enough to join the Confederacy, why that just begs for Seward to annex Central America... Not to mention the ACW come sooner over support for Walker. Or later.
Sorry, but I really doubt it will succeed in the long run. Don't forget the many, many tens, (if not perhaps a few hundreds) of thousands of potential guerillas in these areas; that kind of thing is what caused so many problems IOTL for the Soviets when they invaded Afghanistan, and they were about as decent, probably somewhat better, than that of the *Army. (Maybe we can just exchange PMs instead as not to clutter this thread.
Anyway, I digress; perhaps Walker could become a bosom buddy of the C.S. but I think it'd likely lead to a beatdown of epic porportions not seen since the fabled Masada incident of the days of Rome.