I dont think there is a method out of the box in XmlDocument class to load and convert the encoding. You can try the following, its from the link i gave you, the documentation for System.Text.Encoding:
Encoding ascii = Encoding.ASCII;
Encoding unicode = Encoding.Unicode;
// Convert the string into a byte array.
byte[] unicodeBytes = unicode.GetBytes(unicodeString);
// Perform the conversion from one encoding to the other.
byte[] asciiBytes = Encoding.Convert(unicode, ascii, unicodeBytes);
// Convert the new byte[] into a char[] and then into a string.
char[] asciiChars = new char[ascii.GetCharCount(asciiBytes, 0, asciiBytes.Length)];
ascii.GetChars(asciiBytes, 0, asciiBytes.Length, asciiChars, 0);
string asciiString = new string(asciiChars);
You just need to change it for the encodings you're using. Also you might want to check this question, it has many answers that might help you.