So, as long as I understood I must cross RX to TX ans TX to RX wires within the cable between TTL/RS232 converter and my existing K-Line/RS232 interface . Am I correct? Is it called null-modem if I am not mistaken?
Usually this is true. I've seen boards where the crossing of the RX and TX lines where already done onboard, so it depends a bit from device to device. But in most cases you need to cross them.