How can we patent roducts that are made for smartwatches and other 3rd party devices?

A single product may have features covered by several patents. You can expect wearable electronics like the iWatch, I mean, the Apple Watch, which are produced by tech behemoths to be very well protected. Further, you can search the USPTO's patent database by assignee to uncover issued U.S. patents (and published U.S. patent applications) covering various products. Your ability to acquire patent protection for your ancillary products will depend on whether what you claim is your "product" is patentable subject matter and whether it is sufficiently new, useful, and nonobvious over the prior art. There is nothing inherently UNpatentable about your device being used in complementary or ancillary fashion to another device, but the patentability analysis of your invention would take the scope of protection afforded to the original device into consideration - e.g., disclosures in patents covering the Apple Watch may preempt what you are trying to do or may create an infringement risk.

It is very advisable to engage IP counsel to evaluate your technology on patentability and related infringement matters concurrent with your product development efforts.