Use this Online RSA Key Converter to convert a XML RSA key into PEM RSA key format and vice versa. Reference for RSA Key formats. If you like this online rsa tool, please share the love. Then someone like you will find it handy.
This online RSA Key Converter is built on top of BouncyCastle C# libiary
If you are interested how this converter works behind the scenes, check out the gist here