When applied to Master's degree programmes, the term initiële is used to mean that it follows on directly from a Bachelor's programme and is fully subsidized by the government.
A post-initiële Master's degree programme is separate and is often pursued after a period of employment and/or by students who have transferred in from other institutions. It is not subsidized.
Because the distinction has more to do with funding than with level, it might not be at all relevant outside the Dutch national context, except as it affects the tuition fee.
Consider not even attempting to translate the term. Instead, describe the features of the programme. i.e. making sure that the financial aspect is explained for non-Dutch readers if it is crucial to the text at hand.