Just add a pound or two of boiled DME.
Or, if time is of the essence, steep a 2 pounds of carapils with a pound of 2-row and add it into the carboy. It will add very little additional fermentable wort, but will add to the maltiness so the everclear doesn't dominate the thin beer.
And, honestly, I'd rather add vodka because it is a purer, better distilled, product IMHO.
And, honestly, I'd rather just add more DME.
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