I just got approved for a MasterCard. It was only for 300. Can I increase that amount by adding my own extra money to it
You can send money to the bank and create a credit balance, but you cannot increase the credit limit. So if you send them $100, you can then charge $400 in one transaction.
That said, I see no practical benefit to doing this. If you want to buy something that's $400 and only have $100, just pay $100 in cash and charge the other $300. Or save up and pay in cash :)