CompleteFTP can authenticate users by using an external database table containing user names and their hashed passwords as described here. A SQL statement needs to be crafted to extract the data for a given user from this table. As CompleteFTP uses hex strings for password hashes, a hex string must be returned for the hash.
If password hashes are stored as byte arrays, they must be converted to strings like this
SELECT CONVERT(VARCHAR(40), pwd_hash), 2)
Make sure the VARCHAR length is long enough to store the hash - for SHA1 or MD5, you need a length of 40.