We use another database (event DB) that we use our member's data and we use their member ID or member's Number relate them. Does AMember place a MemberID field that can auto-increment starting from a value the Admin sets?
aMember has an user_id field that is auto-increment and you should not rely on values in this field. Easier way would be to create new field in am_user table, say "eventdb_user_id" and put third-party id into this field.