Hello friends,
Fortunately the OpenType font format allows for glyphs in either Type 1 or TrueType format. So all you have to do is make sure the font conversion program you use doesn't automatically convert the glyphs to TrueType format during the conversion. Fontlab's font converter, TransType Pro, will even add some OpenType features during conversion if the appropriate glyphs are present.
Best regards
Elvish Holic