"Cette boîte servira de chaise."
Traduction :This box will serve as a chair.
Where I live, you would be much more likely to hear be used as.. so a pat on the back for you :) Saying an object can serve as such and such is excellent english, but one (sadly) hardly hears it here (Australia) It may be different in UK or US.
We would often say We'll use this box as a chair, rather than the passive voice, but that is not the DL sentence. I am writing that just FYI (for your information), so you have another expression you can use :)