You'll have to use your existing xbox live account as the main account for the windows phone (I think it will require a hard reset to do so now) if you want to be able to sign into xbox live on the phone. I could be wrong but I don't think there is any other way to add an xbox live account other than the primary phone account.
You don't have to sync anything from the main account for the phone, just uncheck the box next to "contacts" (settings > email+accounts) and it won't sync your contacts. My main Microsoft account doesn't do anything on my phone... not even email... but it is connected to my gamertag, music pass, and store purchases so that's why it's the main account.
05-31-2013 09:36 PM