I think they can push to add additional 300M but I doubt they will do it for the sake of this bet.
Bitcoin is showing a slight price correction from ATM and maybe they will use this opportunity to purchase more Bitcoin. Also they might just teasing the public to create another hype on Bitcoin for new ATH since they are the one who will benefit on it when Bitcoin price pump.
A country that already invested 700M will have no problem to add more.
They may/not achieve it. If they remain committed and bitcoin price turns out in their favor, that's if bitcoin price can hit $150k on or before December 25th, the worth of their bitcoin holdings of $700 million with the additional ones that they will be accumulating might push the price of their bitcoin portfolio to $1 billion.
I love such bet because it will motivate other countries to follow the footstep of El Salvador bitcoin accumulation strategy because it's very effective on gradual increase of your bitcoin holdings.