Extending JSP Custom Tags

Question

How do you extend an existing JSP custom tag?

As you know, a custom tag consists of two parts, an implementation class and a TLD file. I can extend the parent custom tag's class, but how do you "extend" its TLD file? One obvious solution is to cut and paste it and then add my stuff, but I wonder if there's a more elegant solution like the way you extend a tiles definition in Apache Tiles.

Thanks.