How to incorporate Ascorbic acid and diastatic malt while kneading?

As to the Vitamin C, I add it when I mill flour from the berries, so I am sure you can do it then, or at any stage so long as it is in the dough prior to bulk ferment.   I think the same is true as to the Diastatic Malt, but don't use that much.  

I also add the ascorbic acid in powdered form to freshly milled flour then hydrate the flour and let it autolyze. The theory is it promotes gluten development. Like barryvabeach I haven't used malt. The C doesn't appear to negatively affect the action of yeast and may even enhance it.

I use diastatic malt in some formulas because the author recommends it. Final mixes that incorporate a large percentage of preferments, like levains and poolishes, may be somewhat depleted of nutrition. The diastatic malt introduces maltose, which feeds the LAB. The LAB break down the maltose and leave fructose as a byproduct. The yeast cannot break down maltose but love fructose so it is a win-win all around.

Grain mills use diastatic malt to regulate the falling number factor in new lots of flour. This is done so that every lot of flour performs the same as the lots before. This is why you will see diastatic malt listed in the ingredients.

It is hard to say how much to use since I wouldn't know if your final mixes are depleted, and if so, by how much. One of my formulas that uses a lot of preferments add back 1.8% of diastatic malt. It is added to the final mix along with the preferments and salt.

Jim

blend lemon juice with the liquids.

Dry powdered malt gets whisked into the flour before wetting it.