There are PCIe to U.2 adapters: PCI-e x4/x8 to SFF-8643 cable or PCIe cards with U.2 connectors on them (drives mount to card). There are also cards that connect to NVME m.2 slots and connect to a SFF-8643 cable.
Be careful since these are PCI 4.0 4x drives so the dual drive cards should all...