NVS maintainer here. NVS does not recognize a version string such as “lts/boron” because that’s not the way NVS version strings are designed to work. In a two-part version string, the part before the slash is supposed to be one of the remotes configured in settings.json. (A remote can name an alternate Node.js download location such as “nightly” or another fork of node such as “chakracore” with builds published in the same format.)
So to add an LTS version of node, the correct command is nvs add node/boron, or just nvs add boron (since node is the default remote).
Note it is explicitly not a goal of NVS to support all of the same command-line syntax as NVM, because the two tools have many different capabilities. But I suppose I could consider recognizing “lts” as a special-case, to ease migration from NVM.