Hook, Filter and Action Question

I am trying to develop a plugin for the responsive themes like the one I did for the non-responsive themes:

https://delimewebsiteservices.com/downloads/ppt-select-lowest-category/

It needs to modify the function:

function BUILD_FIELDS($fields,$data=””)

in class_white_label_themes.php

The return for the function looks like this:

return hook_add_build_field($STRING);

I have not been successful at finding the correct hook, filter and/or action to modify this.

Can anyone point me in the right direction?

Thanks.

Jim