It's illegal, called "reverse passing off", and violates 15 USC 1125.
Why? 4 policy reasons -
1) reverse passing off deceives the consumer into falsely thinking the product originates with you or under your control and is typical of your brand rather than the original name brand,
2) reverse passing off deprives the true brand owner of goodwill from sale of properly branded merchandise and deceives consumers into thinking your brand is equivalent in quality to the original when if you previously or later choose to use a different source it most likely will not be [you should not be allowed to use the other product to build a reputation based on their quality and then be in a position to switch to lower quality without customers suspecting it],
3) reverse passing off dilutes the exclusivity of the branded product by falsely implying your brand is an alternate source for equivalent product of different manufacture [particularly where the other product is patented], and
4) reverse passing off puts control of quality reputation of the product into your hands [maybe you just repackage, maybe you dilute with horse pee] rather than the hands of the brand owner and that deprives the brand owner of the opportunity and responsibility of controlling the quality of products bearing the brand.
If you want to repackage the branded product, do not alter it in any way and use the brand name and a proper disclaimer. You will get an attorney to prepare that disclaimer if you have good business sense. Don't try writing disclaimers on your own, unless you are very, very experienced at doing it. If you are here seeking such basic information, you are smart to do so but you ARE NOT that experienced. Just thought I'd put some large bold print, some just large print, and some fine print as this is paragraph is about use of disclaimers where the importance of the information is often opposite of the size of the print, LOL.