It's certainly legal. Since you will be paying this loan back, however, the bank will need to count the payments against you in determining how much you can afford to pay each month (your debt to income ratio). That will likely decrease the size of the mortgage you can qualify for.
It is generally not a great idea. Taking out a loan that reaches or even exceeds what you can get preapproved for means that you will be spending a tremendous fraction of your income on debt payments. The general advice around here would be to look for a home that is well below the maximum value you can get approved for since that ensures that you'll have enough money to pay it back and to handle other financial priorities. Borrowing from a relative further complicates things. You'll be putting yourself in a precarious financial position where your father is on the hook if something goes wrong. That can stress the best of familial relations.