Would things turn around for good if underdeveloped countries start to add value to their natural resources themselves without selling to the countries with good economy?
That's the right thing to do but no country can change immediately their current situation.
Underdeveloped countries has a bad economy and one reason is due to debt, their debt has balloon since they borrow money and not spending it accordingly. In underdeveloped countries also, there's a lot of corruption happening as people usually can easily be bribed especially those who are in the government position so that would not help for a country to grow since they are just basically serving their personal interest instead of the public.