Adding "brew enhancer" (dried malt and dextrose) to increase the size of brew-in-a-bag brew

I did my first brew in a bag a couple of weeks ago (i've done kits previously), and it's turned out great - much better than I expected! The only issue with it was that it produced about 4 gallons of wort, which was slightly reduced when boiling, then reduced again when tapping into the fermenter, and reduced again when transferring it to a clearing container (both times leaving behinds the "dregs"), and reduced again when bottling (again leaving the dregs behind), so it ultimately turned into about 3 gallons of bottled beer. It's lovely, much nicer than the kits, but I'd like to try for more next time.

I was thinking for my next one I would top the intial fermentation amount up to 5 gallons, since that's what my fermentation bin can contain, in order to end up with a bigger final yield. I've got a couple of bags of Youngs Brew Enhancer, which is a mix of dried malt and dextrose, and was thinking of adding a gallon of water plus some of that, into the wort, after boiling but before fermenting.

Questions: 1) how much, do you think? Each bag is a kilo. If I was making beer from a kit i would usually add a kilo of sugar to the liquid malt in the can, but I don't know if that's an appropriate comparison when i'm making my own wort.

2) Is after boiling the right time to put it in? Maybe I should add it to the wort before boiling, then add water to top it up to 5 gallons afterwards?

3) Is this just a terrible idea? Is it going to ruin the flavour?

thanks!