I am of the opinion that you can add anything to tea. You hear me? Anything! Sugar, milk, honey, cream, other teas, pepper, cinnamon, vanilla, fruit bits, rosewater, juice, other teas... Anything.
That being said, I think it is important to taste the tea before you add anything to it. Every single individual tea is different.
My experience with white teas has been that they generally don't require anything, and that many ingredients overpower their delicate and subtle nature. I will admit, however, there are a few white teas out there that can handle (or even beg for) the addition of very small amounts of other ingredients without becoming overpowered.
Anything you add to a tea should be added to enhance the flavor, not mask it. I suppose there is some masking when one "smoothes out" the flavor of a bitter tea by adding milk, but diminishing the bitterness often brings out other flavors that were masked by the bitterness. I guess what I'm trying to say is that the ingredient should be appropriate to the specific tea in question, and should be added in small enough quantities that it doesn't overpower the tea.
The thing is, white tea is so expensive, if you are considering adding so much stuff to it that it no longer tastes like white tea, why not simply drink a cheaper kind of tea (or no tea at all)?
If it is for health benefits, you might want to consider that, while there is some difference, there appears to be a give-and-take between the different teas, so you are probably better off drinking a tea you like than trying to mask the flavor of a tea you don't like with a bunch of additives that may or may not be good for you.
As for how added ingredients affect health benefits, I have no idea. I more consider them a possible side-effect of tea than a reason to drink it. I would imagine that tea without additives is most likely better, since it has 0 calories and all that. But, maybe if you need the calories?