Roles and permissions

Moodle 1.8.4

I'd like to provide a role where a teacher, who is restricted to their own group, can add resources and activities to a course, but can't delete them.

Here's the reason.

I'll create a course in which I'll place mandatory resources and activities. It must NOT be possible for the teacher to edit or delete these mandatory resources and activities, but they should be able to add their own resources and activities to supplement the mandatory ones.

I can't see how to do this by setting permissions for a new role as all I can do is switch managing activities either on or off.

Is it possible to set a permission as I'd like?

Many thanks