If I use RCA to XLR cables to connect the RCA output from audio source to the balanced XLR input in my preamp, does it means the unbalanced audio automatically converted to balanced in transport?
Correct. Starting with an RCA connection and ending with XLR does not magically convert unbalanced to balanced; you need to start with a balanced output and end with a balanced input. Balanced connections require a send and return pair plus a ground from beginning to end.