Many of the older wireless headphones manufactured by Sennheiser were designed before the digital/optical connection became as common as it is today and up until quite recently most audio devices had both RCA outputs and digital/optical outputs hence the headphones were able to be connected directly.
It is not possible to retrofit older Sennheiser models to have a digital audio input as the internal electronics are designed for an analog connection/signal not a digital connection/signal. NOTE: Newer wireless headphone models (like the RS 175, RS 185 and RS 195) do have optical audio connections.
Since a digital to analog converter/decoder is not required for all applications it is not practical (and in fact would be cost prohibitive) to include this kind of a component with all older headphone models. Instead if a digital to analog decoder is needed for an individual set up it would be purchased as an aftermarket accessory. A good quality digital to analog decoder is the OREI DA 34.