Currently we do not have a code example that shows USB CDC Class implementation along with DMA.
When using DMA with manual or Automatic buffer management you are simply using DMA to transfer data from the Buffer to USB Endpoint instead of doing it manually (using CPU). You can refer to the PSoC 5LP Audio code example that uses DMA for data transfer. You can use that as reference to see how to setup the DMA to transfer data from the SRAM Buffer to USB Audio Endpoint (in your case it will be CDC Endpoint). Link to the code example: https://www.cypress.com/documentation/code-examples/ce95390-usb-audio-psoc-35lp
You can also refer to the Functional Description section of the USBFS datasheet that provides more information regarding DMA with Automatic and DMA with Manual Buffer management modes of USBFS.
For more information on how to configure DMA, please refer to the: AN52705 - PSoC® 3 and PSoC 5LP - Getting Started with DMA