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.