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I have a go-to recipe for Honey-Oat Pain de Mie. I'd like to add about 1/2 cup total of combined sunflower and pumpkin seeds. Since I do bake it in a Pain de Mie pan, 
I'll pull a small dinner roll's worth of dough just to make up for the increased overall volume.
Do I need to adjust anything else? Will the seeds affect how the bread bakes?
I think the substitution should be straightforward. From the recipe, the dough should be able to handle that amount of seeds.
My main concern would be with hydration level. Seeds can pull some moisture out of dough, which might make the bread a little dry or stiff. But the type of seeds you're talking about shouldn't absorb that much water, so it may not be a huge issue. (This probably needs a couple tablespoons of extra liquid, roughly within the "error range" of the recipe already.)
I'd "play it by ear" and see if the dough seems drier than usual. If you plan to do this substitution more than once, you can see what happens the first time and add a little water if necessary in future batches. (You could also try soaking the seeds first, but that seems unnecessary to me with these types of seeds -- nevertheless, that can give you a sense of how much water they might absorb if you measure the water and then measure the amount your pour off after soaking, before adding the seeds to the dough.)
The other issue is that you'll simultaneously be adding weight to the dough while removing some gluten (by removing a bit of the dough). Now the bread has to support more weight, so you need to ensure the gluten structure is strong. I personally would consider substituting some bread flour rather than just AP, which will help support the extra weight. (That could also affect the moisture level of the dough.) And/or you might add a "stretch-and-fold" maneuver halfway through the first rise (or maybe two), just to ensure the gluten has maximum development.
Finally, oven spring will be a bit less, so you'll want to wait until the dough is clearly risen enough before baking. You also want to be careful not to remove too much excess dough when substituting the seeds, or you may not fill the pan to the top during the bake.
But these are all rather "picky" things. Chances are if you just did the substitution you suggest without any other modification, things will turn out OK -- maybe somewhat dry, and maybe with the loaf a little denser in the crumb toward the bottom.
