M.Tech usually does not permit external candidates to join as M.Tech courses involve regular attendance, lab sessions and projects. After your B.Tech or equivalent degree most universities would expect you to join as a full-time student. But there are few institutes, where part-time or distance M.Tech is provided with relaxed attendance guidelines- it differs according to the university. In case you are interested in doing M.Tech outside, then verify in particular colleges whether they have that facility, although you should be prepared that most of the good universities give preference to regular students. So, plan accordingly and choose the path that fits your schedule and goals!