Two of the most important things in obtaining a Mortgage Loan Modification is that you have to prove, "how important it is for you to stay in your home", and that you have or will incur a hardship. The most obvious hardship is that you are behind or will be behind in your mortgage payments. The modification you're proposing has to make sense to the bank. Even though they don't need more foreclosure properties, they want to make sure you can afford the new modified payment and they won't have to do a new loan modification over again in a few months.
You'll have to provide a ton of paperwork in doing your Mortgage Loan Modification to prove this, but one of the most important things you'll need is a "Hardship Letter". The Hardship Letter is actually what starts the process. The underlying theme of the Hardship Letter needs to be "how important it is for you to stay in your home".
There's a lot of opinion about what needs to be in your Hardship Letter, but it can't be stated enough, so I'll say it again; the theme of the Hardship Letter has to be to show the bank, "how important it is for you to stay in your home". If the bank detects you're not that sincere or that its not that important to you, you're dead in the water.