Reddit user VeryLosh discovered that it’s possible to convert old Karazhan gear (item level 50) into Sepulcher transmogs via the Creation Catalyst. I believe it will also work with old Grimrail Depot gear.
The player used Wrap of the Fixed Stars (item level 50 back) and converted it using 600 Cosmic Flux into Cloak of Sweltering Flame. The item level was still 50 after the conversion and this method is only useful if you have old Karazhan gear from Legion in your bank. You can watch the whole conversion process below.