No, there is nothing in the UCMJ which requires an admission or confession be made under oath to be entered into evidence. In fact the vast majority of admissions or confessions entered into evidence were not made under oath.
Unless I'm misunderstanding,or misreading,what "jagusafr" said there is,at least,a problem with such a scenario in a court martial.Perhaps at some point he'll respond to my earlier post and we'll if I need remedial reading classes! ;-)