It seems unlikely
Strangely I was just considering the same question. The common answer was NO, although I found someones route, on a website, that headed SW from Ethiopia, popping into Kenya W of Turkana, and then heading into S Sudan and across to Juba. I'll see if I can find the link again.
It will be interesting to hear more reliable information. I've heard somewhere that the Kenya to Juba road is pretty bad and slightly dangerous. Compared to what I'm not sure, but probably the allegedly simple Uganda to Juba route.