can I make a dry ingredient waffle mix with dry egg powder & powdered milk?
You can certainly make a home made dry mix for waffles, but it is difficult to make one that will require only water to use.

Obviously, some of the ingredients are naturally a dry powder to start with:


Flour
Sugar (if you use any)
Chemical leaveners like baking powder or baking soda
Salt


De-hyrdated versions of dairy products are readily available, including powdered milk, and powdered buttermilk, depending on your preference.  These can be had from most normal grocery stores, at least in the US.

Powdered eggs are available, intended for emergency storage, or for camping.  Much to my surprise, you can even find powdered butter on the market, also intended for emergencies.  I cannot vouch for the quality of this product.

With these ingredients you could make a dry mixture which is essentially everything in a waffle batter, dehydrated--to just add water.

I suspect if you are willing to relax the "add only water" restriction that you will get a higher quality waffle.  Most home made pancake or waffle mixes expect you to add butter and water, or butter, eggs and water.  

You can find specific recipes by googling, although pancake mix recipes seem much easier to find.