USB and Thunderbolt expansion cards. Sorted by how many NAS models each is listed for.

PartNAS modelsBuy
QNAP QXP-10G2U3A210Amazon eBay
QNAP USB-U31A2P01178Search Amazon
QNAP QXP-T32P2Amazon eBay
QNAP QXP-T52P1Amazon eBay