Using other peoples packages from within C-based R-extension

Hello,

what if I want to write a package mixed R/C-extension
and want to use code that is provided by other peoples packages?

How for example can I use one of the provided wavelet packages
from within my C-based R-extension?

Somehow I would need to load the other packages and have access to the
functions they provide.
I mean I don't want to use the other packages via R-level, but directly
on the C-layer. Something like shared libs (dlopen and such stuff)
but via R-API.

Is there a general aproach to this, and how to do it?