I have a custom made system (non Drupal) that has a taxonomy like:

Element 1
   Element 1.1
   Element 1.2
   Element 1.3
Element 2
   Element 2.1
   Element 2.2
Element 3
   ...

And I have a Drupal site that would like to be able to:

  1. Use this taxonomy for tagging content, and
  2. Add more terms that the non-Drupal system can use too.

Is there a popular way to sync a taxonomy in a Drupal system with an external system?