I bought a RTX 3090 separately and it works in my P620 just fine.
Part of buying a workstation with an online configurator is that you don't have to assemble it yourself.
Yup, I totally get it. I had to tinker with power cables (the EVGA RTX 3090 required 3 x 8 pin power connectors). Also, cooling - I couldn’t get my hands on a blower-style RTX card, so the GPU just dumps its heat inside the chassis.
I also bought cheap “eBay memory” instead of the more expensive Lenovo offering and had to reseat DIMMs a few times to stop having ECC errors (the memory scrubbing feature is nice).
Those are mobile GPUs. There's nothing in their workstation line card with a 3000 name.