I assume your "Extended ASCII" is actually code page 437:
Encoding cp437 = Encoding.GetEncoding(437);
byte[] source = new byte[1];
for (byte i = 0x20; i < 0xFE; i++)
{
source[0] = i;
AnsiLookup.Add(i, cp437.GetString(source));
}
Beware that this code page is not natively supported by the .NET Framework, so it might not be available on all systems.