I wish there was an easy way in the phone to do this. I've searched for a solution and the best and simplest I've found for only a few conversions from phone to google is to add them in your gmail.com contacts. Then sync and delete the old ones on the phone.
If you have a lot of people that need to be converted to google contacts, I believe you can find an option to export the contacts to your sd card, then reimport them, and it gives the option of making them google contacts. However this will duplicate everything and I dont remember the exact way to get rid of the duplicates all at once. Search forums and you'll find it.