When having the "Make Member" GControl option set to Owners, the moderators are still able to use tempmem power.
Thanks @Pia for your help !
Step 2: Go to GControl power and click the edit button.
Step 3: In the "Make member" box, put "Owner" rank and then click "Ok" button.
Step 4: Save the changes.
Step 5: Tempmem a user while being a moderator.