Yes, @JamesNorth is absolutely right - but you can do this for any group you want so if the ‘admins’ you are talking about is a broader range of members than those who have full admin rights, then it is still simple enough:
- Create a Group for your ‘admin group’
- Make that group not visible to anyone, and not one they can join or leave (so you can manage it)
- Add members into that group
- save it
- Create your category for conversations you want to have only visible to this admin group
- in the security settings (as per James’ comment) you give only members of this group the rights to create/reply/see this content
- post away secretly to your heart’s content
I’ve used this not just for admins but for private groups, and for notifying selections of members about content
good luck