Is nano able to add gpu,e.g., gtx1060 with a pci riser?
No. The built in GPU is wired directly to the memory controller, and the driver is unable to handle GPUs over PCI. The publicly available GPU drivers for PCI are not available in arm64 architecture. I have heard there may be interest in producing arm64 drivers for PCIe cards, but so far as I know this is all speculation and not any kind of official plan.