Currently, partners can offer tiered subs permanent access to 1-3 pre-chosen emote modifications, but the only way to have an option for channel point modifications (either affiliate or partner) is to allow anyone to modify any Tier 1 sub emote, and nothing else. It would make sense, and be easier to explain, if the "Modify one emote" option let you work with any channel emote you have access to, and nothing else.