Adding a Device to an Existing Patented Product

Hi All,
I am in process of developing an add on assembly to an existing patented product. My design does not interfere with the original products design and does not alter it's function. The only modification I will need to do to attach my additional product is drill some mounting holes and mount it to the frame of the product.
Do I still need to contact the manufacture of the original product and negotiate a license deal to use their product? I don't want to manufacture their product I only use it in it's original condition and add my parts to it. I also want to patent the final assembled product and market it. One more piece of info. I have seen the same product manufactured and sold in China under a different name. I understand that the patent rights of a product registered in a foreign country does not apply here in the states. If this is true I should be able to use the Chinese version without negotiating a deal with the patent holder. The other scenario that could be is the product was copied and manufactured in China from our patented design here. Starting to sound like a can of worms to me. Any advise out there?

Thanks

In general terms, you do not need any special authorization to make an aftermarket accessory product for an existing product. You do need to be careful not to market any aftermarket accessory in a manner that makes it appear to have been manufactured or (in the absence of an actual endorsement) endorsed by the manufacturer, and need to be careful that any use of their trademarks is purely incidental -- but if you're reaching that level your product would appear to have sufficient market potential that you should run your marketing plan past your business lawyer.

You can patent an accessory product, assuming it qualifies for patent protection, but you can't patent it as part of another company's patented product. Despite the self-help books on the subject, writing a patent application is rarely something you would want to do without a lawyer. It's easy to make small mistakes that can render a patent unenforceable. You will also want to consider possible alternative versions of your add-on that another company might be able to make without violating your patent.