Your first post is a little confusing... I think the second one clears it up.
1. You have a mini-system in the kitchen that is already hooked up to speakers.
2. You want to run something from your main system's zone 2 to the aux input on the mini system in the kitchen.
3. You are not connecting SPEAKERS to anything - you are connecting the RCA outputs of one system to the RCA inputs of another system.
As jaxvon said, you can use coaxial cable. You need two runs of it - one for the left channel, one for the right channel. You could just use a long audio interconnect (not speaker cable!) but this would be more of a headache then it is worth and likely wouldn't provide good shielding.
Home Depot offers RG-6 quad shield coaxial cable for about $60.00 for 500 feet. Likely far more than you need, but do some math when shopping online and include shipping and think about how you may use it in the future. You can solder RCA connections onto the cable or crimp on F-type connections and use an F to RCA adapter if you would like.
The hard part is getting the cable from one location to the other - once that is done, terminating the ends and hooking it up is easy.