Effect of adding wet ingredients to dry when making bread?
The main issue you will face is the extra mixing allows for additional gluten development.   In some breads, this could create a risk of over-kneading, which could make the dough less workable, more prone to tearing, and more difficult to get the proper rise.

Challah is a basic egg enriched bread, so other than the eggs themselves (which are fairly effective at helping prevent over-kneading) you don't have a great deal of protection.  The worst case scenario is that your bread will be a little flat and crumbly; the best case is that you are still well within the tolerance window and it will be just fine.  

Edit:  I will leave this for reference, but I was thinking of brioche:  in a very sweet, fatty bread like brioche, this risk is minimized.  The fat and sugars in the dough act as barriers preventing the glutin-precursors from interacting as frequently or as easily, making it much, much more difficult to over knead.



The main reasons for the traditional order of dough assembly are:


As you no doubt realize, it is much easier to incorporate the liquids evenly into loose flour, rather than a partially formed dough
The yeast acts on the ingredients present, so sometimes ingredients which inhibit yeast growth are held back until after a fermentation phase
You want time for all of the flour to be fully hydrated--normally, in yeast raised doughs, this is hardly an issue.