You risk oxidation and infection. Plus there's usually no yeast left in commercial brew (it's filtered), so you'd need to add that.
Aside, from it being possible, why would you?
You can make a much better beer brewing it yourself, and probably for less money.
On a much smaller scale, I've spiked common lager with "hop tea."
For this I made a hop tea by steeping hops (e.g., .5 oz Cascade and .5 oz Belma) in a good quart of 180°F water for an hour or so.
Strained the hops out and added 1/2 oz of cooled hop tea to a glass of Yuengling Amber Lager (16 oz can). It tasted very differently and the fresh hop flavor and aroma were out of this world. I'd say it was better.
The next experiment was simmering the same hop bill for a few hours (2-3 hrs) to isomerize the alpha acids and create bitterness as well as flavor and some aroma. This was another wonderful and tasty concoction.
When mixing the potions from experiment #1 and #2 together you can control bitterness, flavor, and aroma in your "hop-spiked" beer. Add to taste.