3 station Nutone IK-25 intercom system in place. Want to add a second inside speeker.

I purchased a 3 station door intercom system IK-25. I have installed it along with a second door speeker for back door. Everything works great and I would like to add a second inside speeker into my hobby room in basement. The supplier I purchased this system from researched if I could use a model IS-68 and was told it should work. I have been trying to make it work but no success. I want to wire it into the inside speeker system. Do you have any suggestions or information on this. Your assistance would be greatly appreciated.

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