The dictionary defines the word,"proliferate" as, "to increase in number".
Perhaps I don't catch the nuance, but how can the U.S., which already possesses a nuclear arsenal, increase the world nuclear proliferation problem by taking a number of nuclear weapon that it already has in it's inventory and
converting them to bunker busters?
Now are we strictly using weapons we already have and converting them, or are we producing more nukes (ie increasing the total number of nukes we have)?
That is the true question of proliferation.
If its the first option then no we are not increasing proliferation. If the second part is true than yes we are increasing proliferation.