Hi Doug, welcome to MoI!

> putting a curved cushioned surface on a button such that
> the top resembles a membrane that has had pressure
> applied from bellow.

It's a bit hard for me to really clearly visualize what you are describing here - do you possibly have any image of that kind of membrane shape that you could post to give a clearer picture?

One method that may help is if you model the membrane more directly all by itself as a separate surface, something like this (here using sweep with one curve as the profile and the other as the path):



That will make a broad curved surface like so:



Then you can use either the Trim or a boolean command to combine those pieces and have the outside of that broad extended sweep piece get cut away. If you've got 2 open surfaces that form a volume in their inside area you can select them and run the boolean Merge command to get this type of result:



So note there that this technique does not try to construct a surface that follows directly along the boundary of your starting piece - instead an extended simple surface is created and then it gets attached to your other shape by a cutting process. Often times this technique of using intersection between pieces to generate the final edges is a key construction technique for NURBS modeling, rather than trying to focus only on constructing surfaces that follow directly along some already existing edge structure.

Also see here for some links to more descriptions of this kind of rounded end cap technique:
http://moi3d.com/forum/index.php?webtag=MOI&msg=4851.3

- Michael