Hi, remember that custom decorations are currently not accessible outside of the part manager.
That means that it's currently not yet possible to use them in the workshop. Scrubs is working on this part.
For now, you can already create custom parts in the part manager.
Also scrubs told me he has no idea when you will be able to use them in the workshop, and told me maybe before the end of the year. He also said he probably has like another 50000 lines of code to write