The PC only has a single M.2 slot for an NVMe SSD, so you can use the HP SSD 2TB PCIe 4x4 NVMe G11 as an expansion, but only as a replacement or exchange for the existing SSD, not as an additional second SSD. A second M.2 slot is not available and there is no bay for an additional 2.5" SSD or HDD.
Hello!
I've been using the Pro 400 Mini G9 regularly for six months and to be honest I didn't even realise it had a fan, it works completely silently. It sits horizontally on my desk, under the monitor. If you really do have the same device, try out what happens when you place it horizontally and well ventilated. If it still makes noise then, it is a case for the guarantee, if not, you should think about the positioning, constant overheating is certainly not good for the device.
Kind regards!