The reason modpacks exist is so it's exactly that selection and configuration of mods. Being able to add mods to modpacks would destroy the purpose of modpacks
If this addition is requested, it is good that it is useful to add it. We would like to be able to add more, but it is not mandatory.
But it's always good to be able to add more! 
Thank you for your understanding!(Sorry my english is not perfect)