You simply need to create a symbol under the Racks folder with the same path and name as the Blocks symbol. i.e. if you have:
My Libraries | Blocks | Acme | RR-9000
you should create
My Libraries | Racks | Acme | RR-9000
Note that paths and names are case sensitive. Note also that the Generic Components library in Racks has hundreds of high-detail components from we which we ourselves generate rack layout symbols.
I hope this is helpful. Kind regards,Rob RobinsonStardraw.com