RCA to coaxial/optical

Hello, i need help if is it possible to conver 3 RCA jacks (5.1 audio) to digital spdif (coaxial/optical)output. The point is that I have ROCCAT Kave xtd 5.1 digital headphones with real 5.1 sound card but it has onla 3x RCA 3.5mm jack output for 5.1 audio but my home theater receiver have only...