Dear Miles,
Yes, the idea is to see the density Matrix MPO as an MPS with two site legs per site; in this way for instance an MPO with d local dimension can be seen as an MPS with d^2 local dimension. Thus, my answer in brief is if in Itensor there is a function that permits to extract from an MPO with d local dimension its MPS representation with d^2 local dimension. In few words, if It Is possibile, given an MPO with one up leg and One down leg per site, to extract an MPS with two down legs per site (the up leg of the MPO becomes the other down leg of the new MPS).
I apologize if the answer is a bit confused,
kind regards,
Francesco Perciavalle