The hifi dac is not activated unless impedance is higher than 50 oHms.
It uses the regular onboard mode.
You would need to find a RCA to RCA (male to female) adapter (or more like an extension cable, as a straight non changing (basically a tiny extension cable) adapter really doesn't make sense to be made normally) and plug that in by itself, then plug the main cable into it afterwards. The "extension" cable with an open connection would act as an infinite resistance (external) device.
Other things which can work (but are bulky) are iphone y-splitter adapters (which combine headphones and microphone cables into one plug), if that makes sense, or a regular splitter (which splits up headphones and mic into two different lines--usually used for computer connections where there is a microphone port).
Something like this would work for you.
(for some reason I can't post the walmart link but try htt `s`:`//https://www.walmart.com/ip/C-E-3-5mm-Stereo-to-Dual-RCA-Audio-Adapter-Cable-3-5mm-Male-to-Dual-RCA-Female-Red-White-6-Inch/775924310
and remove the ` please.
This would also work but it's going to be a lot longer length.
https://www.amazon.com/LASUS-Stereo...ag=hawk-future-20&ascsubtag=UUacUvbUpU6346004