You could put more fabric into the kilt by taking a piece from the part of the fabric which will eventually be cut off and using that to create the under apron and part of the large pleat beneath it - or make the join wherever it is going to be easiest for you.
You are not limited by whatever fabric you have in one continuous piece.
Just be sure to check that the piece you cut off is a perfect match. I have seen fabrics where the selvedges are at different points on the pattern, which then brings up the difficulty of to hem or not.
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