Having done a bit more research I have found this page concerning Awards and Promotions on a website regarding Werner Voss.
The website states that, as would be standard, all soldiers started of with the rank of "Soldat" or a unit based equivalent.
It also states:
Furthermore German Soldiers progressed through the ranks at a very slow pace. Typically a Soldat would not be eligible for promotion to Obergefreiter until he had been in the Army for six years.
Given that wiki page you quoted mentions that the standard conscription period was 2-3 years it would appear that NCO's of a rank exceeding Gefreiter (Obergefreiter and above) would be drawn from the pool on Long-Term Volunteer Enlistee's, as I mentioned in my comment.
I have found the following quote on greatwar.com, which also states that only career soldiers could be promoted.
A private in the German Army made about $5.10 US per month(30-
day month), whereas a US soldier made 30.00/month w/an extra
$6.00 if on "foreign" service. This was comparable to other
European Army pay, and it taught the soldier to spend his money
on necessities and hardened them for tough times in the field.
Promotions were reserved for the career soldiers. Seldom did a
two year recruit receive any rank. The unit commander made all
recommendations for promotions when a vacancy appeared, which was
then approved by the regimental commanders. One year volunteers
with excellent records and at least 9 months service could be
Elsewhere in the article it states that on the outbreak of war all conscripts were obliged to serve until their 45th birthday (A reasonably standard measure, the same happened elsewhere). This change effectively put them on a par with the long term volunteers, as far as term of service is concerned.