For example:
I am a CIA agent disguised as an English teacher.
I wonder which is correct.
P.S.
What words can you insert after A?
I am A (who or who have ) disguised as B.
I am A (who was or who is) disguised as B.
Example is fine.
btw, I’ve always suspected Yutaka-san was a CIA agent disguised as an English teacher…
I’m not sure what your question is exactly. But ‘disguise’ can be active or passive.
He disguised himself as a CIA agent.
His friends disguised him as a CIA agent.
He was disguised as a CIA agent.
These are all correct. Does that help?
Can I say “He disguised as a CIA agent.”? ( no object )
No. “He disguised as a CIA agent.” is wrong. You need an object, or you need “is” before the adjective.
Some past participle (passive) forms are also adjectives. “Disguised” is one. Is that what’s confusing? Look at these examples. Can you see which sentence uses an adjective?
Helen was having a bad day. She was cold, she was hungry and she was disguised as a CIA agent.
Helen was disguised by her sister, who was a Hollywood make-up artist.
This chicken is delicious. Was it cooked in the oven?
Is the chicken cooked? I’m starving.
The walls in my apartment are all painted. I don’t like wallpaper.
Jim does the plumbing, Matt does the electrics and any walls are painted by Bob.
The street is closed on Mondays for a street market.
I can’t buy any vegetables because the shop’s closed.
Correct would be:
He disguised himself as a CIA agent.
He was disguised as a CIA agent.
A disguised B as a CIA agent
A disguised B as A disguised as a CIA agent
Umm. He disguised her as a disguised CIA agent; she was a CIA agent disguised as an MI6 agent. She was an English-teacher-turned CIA agent.
love the “an” MI6 agent! Next, you´ll be joining the ranks of those of us who still say “an hotel”. Bliss!
I did not know that MI6 agents have disguises. I just assumed that they were all James Bond.
“If horrific was pronounced ‘orrific’ and historic was pronounced ‘istoric’ then it would be appropriate to refer to ‘an istoric occasion’ or ‘an orrific accident’. In the 18th and 19th centuries, people often did pronounce these words in this way.”
‘der gefangene Spion’ and ‘die gestorbene Mutter’
If ‘disguise’ is a transitive verb, like ‘fangen’ in German, its past particle form will have a passive meaning. But in ‘a CIA agent disguised as an English teacher,’ It seems that ‘disguised’ has only the meaning related to the perfect tense. This is why I feel the expression is strange.
The past particle form of sterben, ‘gestorbene,’ does not have a passive meaning. It is only related to the perfect tense in meaning. ‘Sterben’ in German is an intransitive verb.
You appear to be an expert in European languages who is disguised as a student of English 
I deleted an earlier contribution because it didn’t make much sense (which is sooo unusual for my comments).