Actually, that operation is doing it on an object by object basis (aka looping over every object in the PDF and converting it as requested). Your software would need to do the same - and I am pretty sure that iText(Sharp) does NOT offer such a feature.
If you were to license the Adobe PDF Library (either the C/C++ version or the Java version), it includes the same "Convert Colors" feature that is provided by the Acrobat application.