Seems there's another category, the ngakpa who is ordained as such. At least this exists in the Nyingma.
I think its going to be a case by case analysis when talking about ngakpas who also happen to be western--
just the same as it is with those ngakpas from traditionally Buddhist countries. On top of that there are
various lineages of ngakpas and their style of practice varies.
Quite a few are practitioners of Dudjom Tersar, or Longchen Nyingtik, or Chang Ter; but then there are
dratsangs that practice several lineages and follow distinct traditions.
It isn't actually a ordination, like the ordination to become a monk/nun, it is an empowerment. You can call it an elaborate Samaya if you like.
There is often formal ordination based on tantric samayas.
There is a way of becoming a ngagpa through taking Tantric Vows in the process of empowerments attached to certain sadhanas, which is what I assume you mean. Here's a summary I found:
''For these reasons there are many renderings of the dam-tshig bcu-bzhi according to different lineages of transmission and these are subject to countless interpretations depending on the teacher, the student and his/her stage of developoment and other circumstances.
Although the avoidance of the fourteen root downfalls are the 'defining' vows of all ngakmos and ngakpas, how they are received also varies between traditions, lineages and teachers. Some, like Karma Rejung Lama , or some students of HH Penor Rinpoche, clearly receive a specific ngakphang ordination within which the vows associated with the 14 Root Downfalls are the centrepiece. Others, though, receive their commitments through the damtsigs associated with the receiving and practice of sadhanas but without a formal rite of passage into the ngakphang.''http://www.nyingma.com/artman/publish/c ... x_35.shtml