add_filter

Can you use add_filter on a plugin function?

In other words the plugin has this function that I would like to change, but not edit the actual source:

function plugins_function(){
echo 'do some stuff here';
}

I would like to change it to something else using add_filter() like so:

fucntion plugins_function_custom(){
echo 'do something else here instead';
}
add_filter('plugins_function', 'plugins_function_custom');

Any help on this would be great!