There are other people here who are more versed in buying extra seats than me whom I hope will fill in such details, but it is technically possible to buy an extra ticket for a passenger who either, for example, needs extra space for a medical condition (like having their arm in a brace), or is too large to fit in a normal seat, often euphemistically called a person of size. But that's not what you want to do, since you want to retain your ticket that comes with the cruise, presumably because it is less expensive than buying your ticket yourself.
On the other hand, you seem to be willing to spend a large extra amount to improve your seating. So, here's another idea: can you cancel both Viking-issued seats and purchase another Premium Economy for your husband? Might it make financial sense to do so?