So i have external Digital Audio Converter with 2 channels where i get signal to my active front speakers and sub. But i wish i could get signal simultaneously to my rear speakers from motherboard to create 4.1 surround sound.
Depends on your hardware some motherboards have multichannel outputs. In case your motherboard supports this, there should either be a digital output that supports multichannel sound. Or several normal outputs that you can directly connect to the active speakers.
If your hardware does not support this I'm afraid I cannot help you.