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.