You cannot on that account because the only way to do that is by taking advantage of the Early Edge promotion but you need to turn in a working phone for that. Your only option is to open up another line on either Edge or 2 year contract.
I wouldn't. I added another line, to upgrade and did it under the Edge program. I have had nothing but problems since. I'm still being charged for the Edge phone I don't even have anymore. 1st they said I didn't have to send it back, then I did, then I didn't but am still being charged, after I sent it back.
They are so screwed up right now. I would just wait it out or see if another carrier will buy you out of your contract.
You can add a new line, and get a new phone by 2yr contract or by edge agreement (you would NOT have to return any device). You would continue to pay the same monthly access fee for the broken phone for the remainder of that contract, plus the fee for the new line. You would be looking at double payment for 6 months.
IF you are eligible for an EARLY EDGE - DON'T! Your current device would have to be returned in good working condition or else pay a $300 non-return fee. You may want to consider borrowing or purchasing a used device until you are eligible for your standard upgrade.