This is done with the NAMESPACE file and also noted in the DESCRIPTION file. There are a few ways to import a function in NAMESPACE, but the simplest is just importFrom("[PACKAGE_NAME]",[FUNCTION_NAME). Then, in DESCRIPTION, add the package name to imports.
See this very good tutorial from Friedrich Leisch.
http://cran.r-project.org/doc/contrib/Leisch-CreatingPackages.pdf
