According to the warranty then no, you can't add anything.
According to Physics, whatever load you do add will just add stress to the frame, eventually causing it to fail. The more you load, the more stress, the faster it fails. Less load, less stress, longer before it fails.
Unfortunately, nobody can say for certain how much load that is, or how long it'll take it until it fails, as it's a combination of too many factors to predict. (Such as the amount of vibration, the angle of the load, the strength of that particular piece of steel, or that particular weld, etc...). There are ways to reinforce, but even with that all these variables still come into play.
Better option is to just not do it.
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