It's most probably my awefully horrible English here. Awefully horrible... Damn, I don't know if these 2 even work together, like at all. In any case, why is everybody here translating "will be working" as работаю? I mean he used a future tense there, right? Meaning he's not working there yet, but will soon, at least to my mangled understanding. The Russian translations say he's already working there.

With that settled here's how I'd say that:

Компания, в которой я буду работать, имеет офисы в России. @chahoub где is also possible there as a clause connector, so to speak or... whatever. Nice job. @xXHoax в чём is not however, it doesn't make sense. Your other points are pretty spot-on though!

There's one thing that I think is worth mentioning. If I was having a friendly converstion, I'd most likely express the idea like this:

У компании, в которой я буду работать, есть офисы в России. That first one I'd reserve for more oficial situations, but that could be just me, so take it with a huge grain of sault. I don't think there is really "the best way to say this" however. There are just... ways.