Found an answer.
M.2 B-key pins 29, 31, 35 and 37 are not connected on both Rock B+ and Rock 5T according to PCIe interface docs. Same goes for SATA pins 41, 43, 47, 49.
One can use any M.2 B-Key to USB adapter like SD Card example above since pins 7 (USB4_DP) and 9 (USB4_DM) are connected to CPU’s USB2 HOST0 lane. However you will not go faster than 480Mbit/s so anyone planning to upgrade 4G module to 5G later should take this into account.