Thanks for your response Gary, but I think that you misunderstand what I mean...
I am trying to append a new path to the end of include_path - the path PHP uses to find files when you call include(), require(), etc. In my case, I am trying to set the variable in a .htaccess file using the php_value directive.
My problem is that the include path has already been set somewhere else (for PEAR) and I don't want to over-write this. I would also prefer it if I could use include_path as variable (as per my original post) instead of hard-coding the PEAR lib path, in case my host decides to change it.