Hexadecimal to Json format converter?

[Muhammed Zakir]

Hello!
I backed up private keys of 3 addresses from my Android wallet. I think it is in hexadecimal format. When I am importing private keys to Bitcoin Core, its saying "Invalid private key". Maybe it is because backed up file contains 3 private keys and I don't know how to get one of them from them or maybe the private keys are in hexadecimal format and I don't know how to convert it to Json format. So please tell me any way to import private key to Bitcoin Core and also please tell me how to delete or at least hide Bitcoin addresses from Bitcoin Core.
I am using Andreas Schildbach's Android Wallet.
Kindly,
        Muhammed Zakhir