Getting back to the original topic, you can certainly add more malt extract to raise the ABV. The only issue is that of balance- you need to add more hops to balance more malt.
Think of it this way. You made 1 gallon of spaghetti sauce, but just found out that you need to take 3 gallons to a party. You could add two gallons of tomato sauce, but then it wouldn't be the same spaghetti sauce at all. You'd need to add more oregano, garlic, basil, and so on so it's the "same".
The same is true of your grainbill when you increase the fermentables in your beer. Yes, you'll get more alcohol. But you'll also get more malt flavor, and if you don't balance it out with more hops, it won't be the same beer.
The best way to make a higher ABV beer is to start with the right recipe in the first place, instead of boosting the fermentables in a different recipe.
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