Llenaré de oro el cuarto en que estamos.
Can we say "llenaré de oro el cuarto en lo que estamos", although it seems a little redundant to me? Thanks a lot. 
2 Respuestas
Adding "lo" would make the sentence grammatically incorrect. It's fine as it is.
(Sorry for the original response. I skimmed your post and misread it.)
Llenaré de oro el cuarto en que estamos. --- I will fill with gold the room that we are in .
"llenaré de oro el cuarto en lo que estamos" --- Now here is different. The sentence looks incomplete.
First there is no capitalization at the beginning which makes one think that it is just a fragment of a phrase. There is no period at the end either.
Therefore, I could assume the sentence means "...llenaré de oro el cuarto en lo que estamos... (comiendo?, dándonos un baño?, amándonos?) --- "... I will fill the room with gold while we are (eating? making love, etc.)
It could mean anything when we overlook punctuation and capitalization.
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