Auto assign a category to a custom post

Hey guys,

I’ve got a set of categories such as:
– Item A
—Item A-A
—Item A-B
—Item A-C
– Item B

These are defined in the standard categories section. Now, I created a new custom post which has the identifier “custom_wppost”. I do not want to create a separate taxonomy for it but rather auto assign “Item A-B and Item B” categories to posts made under this custom post. Instead of the users having to select the categories, these categories shall be assigned automatically.

How do I go around with this? Any help is much appreciated.