> Can I do:
> У фирмы где я буду работать офисы в Россию
(in addition to said above) you can omit "has" thing here (translated as "имеет" or "есть" above) ang get:
У фирмы, где я буду работать, офисы в России. This ending is correct here. But this requires more context. You should prefer variants above with "есть" in many cases. For example it's ok to make such omitting in answer to question about this firm or something related to it. Russian language do not like "is" and "has", but at some level they are required.