Thanks for the question. Basically, we don't have that feature right now, because many technicalites of C++ make it quite a lot of work to add. (And there are some design questions about ITensor itself too that we need to think through.) But tensor slicing is high on our list of features we want to add, and plan to add.
So for now, please do an element by element approach. In some cases, such as two-index tensors, you can also construct an ITensor from a matrix using the matrixITensor function. So this could be helpful for you.
If you're really stuck or find this kind of step to be a bottleneck, please let me and Matt know, and we could definitely help you craft some custom code which works on a lower layer of ITensor to help your specific case.
Finally, we think adding tensor slicing support will be much easier and go a lot faster in Julia, so we plan to explore it in the new ITensors.jl first.