the only way i can think of is to back up both wallets and import the back up from terracoin wallet into the bitcoin wallet and send them to your bitcoin address. its pretty complex and could cause alot of trouble if done wrong. it might be worth pming vm1990 he charges a fee but might be worth it
Just backup everything first if you do do this then you have the originals all in one place if it fucks up
id say back up to the harddrive and a pendrive or 2 to be safe. if it messes up you could be stuck redownloading both blockchains 2 or 3 times or even loosing the coins if there not backed up