Short answer: you can't.

Everyone is limited to 300 balls for their tower. Even though their cloud may show more balls, that's the total number of balls they have collected. They're still limited to 300 balls in their tower.

Long answer: The limit had to be the same for everyone worldwide to make the leaderboards fair. 2dboy chose 300 as the limit, as more balls caused people's computers to slow down too much.

Rant: The clouds in the corporation show their total balls (even though they're really limited to 300) which causes this question to come up again and again.

Cheater's answer: you can if you hack the .exe but I strongly disapprove of this and it is liable to slow your computer down to a crawl anyway. Instead why not play the sandbox level?