Do you want to make them thickier from a mesh perspective, or do you want them heavier from a cloth perspective?

in the latter case (not mentioned yet), the way is to select the vertices, put them in a new dynamic group for the cloth, and assign different cloth parameters to it. Especially a higher density, in your case.

This is the way to make multi-material clothing, like dresses from linnen with leather stipes. Or lace/latex patchwork clothes.

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Usually I'm wrong. But to be effective and efficient, I don't need to be correct or accurate.

visit www.aRtBeeWeb.nl (works) or Missing Manuals (tutorials & reviews) - both need an update though