Took me a while to find how to get there. When you wrote SDK, I first thought I had to open it in MXUXpresso. (Maybe that was what you meant, I just couldn't find that example there.)
For reference: I opened the µVision Pack Installer, selected my MCU Family (LPC54618), then activated the Examples tab, then copied the cmsis_spi_dma_b2b_transfer_master example. And after opening it, the functions were there just as Aldo said. (pin_mux.c is under Board support).
Thank you so very much!