Can I add a specific line item that's visible to only admin?

No, you cannot make line items that is visible to specific users.
However, you can configure settings in the User Management where you allow certain user profiles to access the Line Item block.
Please follow the below steps:

1. Log into the CRM.
2. Go to the Main Menu.
3. Click Settings.
4. Select Profiles under the User Management tab.
5. Expand the Inventory menu.
6. Look for Items.
7. Make the desired changes.
8. Click Save.