Extending Natures

I have a situation where, depending on what the
user selects on my project creation wizard, my
nature may extend nature "a" or nature "b". How
can I do this? It seems like all I've seen is
how to specify which natures my nature extends
in my nature.xml file and I don't want to specify
that it extends both "a" and "b".

Also, is there a way to change what natures my
nature extends? For example, after the project is
created, via a system action, I may want to change my
nature so that it extends nature "b" instead of nature
"a". How can I do this?

Thanks,
   -Joe-