You can't transfer purchases or combine Microsoft accounts. There are hacky, long term workarounds, I'm in a similar situation.
What you can do is rename your existing Microsoft account (that uses a Gmail.com address) with a new Outlook.com address. If you already created a separate Microsoft account using an Outlook.com address you want, you will be unable to instantly delete it and rename your existing account to it. You will have to delete the new Microsoft account, wait out the deletion period (I've seen nine months and twelve months, not sure of which it is), then rename.
If you rename a Microsoft account currently registered as the primary account of a Windows Phone, you will have to hard reset the device (this is my dilemma, I cannot rename the account because I wired up my mother's Windows Phone to use my existing address and don't wish to trouble her, especially with the nonexistent backup situation currently).