Can temple clothing be modified with a piece of velcro to help hold the garment on?
I’m not sure, though it seems adjusting the size to fit more properly would be the first step to attempt, whether the garment is too loose or too tight. Modifying the garments is expressly forbidden in the area of altering the dimensions, however I’m not clear at this point whether adding velcro would be similarly forbidden. A valuable resource here would be your bishop or stake president, both of whom are better equipped to answer your question.
Also possible is to order custom made garments to your required dimensions. Simply have a seamstress measure you and write them down, then send those measurements in and from them, custom garments can be made. They are a bit higher in price, but not much more than regular styles.