You can say "expose smb as [noun]". However, I want to somehow add a verb to it. How can I do it? "Expose smb as doing smth"? "Expose smb to do smth"? For example, "The investigators exposed him as taking bribes from pharmaceutical companies" (don't suggest to rephrase it, it's not what I'm asking).
To start with, "expose (somebody) as (doing something)" is actually not grammatically correct (though occasionally people do say things like that, even though they're technically wrong).
There are generally two ways to use the verb "to expose":
- to expose (noun)
- to expose (noun) as (noun)
The first means to expose something (some behavior, or secret, etc) to public scrutiny, and the second is a special form of the first that means to make public the (secret) fact that one thing is (equivalent to) something else.
However, as you've probably noted, both of these forms only take nouns. Therefore, you can't say "to expose (somebody) as (verb form)", because the "as" form is ungrammatical unless both arguments are nouns.
So what do we do? Well, the trick here is that we can convert a verb phrase into a noun, and then we can use it with the first form (not the second). For example, if the action we want to talk about exposing is "to take bribes", this can be converted into a noun by using the present participle ("-ing" form): "taking bribes". Then you can say:
The investigators exposed taking bribes
(A verb that's been turned into a noun in this way is called a gerund)
And this is grammatically correct. But wait, we've lost the subject of the thing being exposed ("him"), which is really the most important part, right? How do we get that in there too?
The answer here is that now that "taking bribes" is a noun, we can add a possessive to it to attribute it to somebody: "his taking bribes"
The investigators exposed his taking bribes
Note that there's no "as" in this, though, and the person in question needs to be put in the possessive form in order for it to work.
You might ask: Why can't we use the gerund trick with the second form of "expose" above to say "to expose (someone) as (gerund)"?
The investigators exposed him as taking bribes (wrong)
The reason this is wrong is because the "expose A as B" form says that A is equivalent to B, that is that they are the same thing. However, what you've said above is that "he" (a person) is the same thing as "taking bribes" (an action), which can't be true (a person and an action are completely different kinds of things, so they can't be equivalent). That's why the "as" form won't work with the gerund trick, unless you're actually talking about exposing one action or activity as being another:
The investigators exposed his hobby as knitting (this is ok)
(Though why investigators are investigating/exposing somebody's hobby I'm not really sure)
I guess what you mean to ask is whether it's possible to use an -ing phrase after the structure, expose sb as. well you definitely could say something like that, However, whether it's going to sound natural or not is another story, how do you go about figuring that out? you could utilize a corpus of contemporary English. I looked up the structure expose sb as -ing in the corpus and very few results turned up indicating that it's not common among native speakers to use an -ing phrase after as. you could use a noun phrase after the structure though:
they exposed him as pretending to be someone he's not ==> they exposed him as a phony. OR they exposed his pretending to be someone else
the one above would be a gerund phrase, so you could use a gerund phrase as well, but you'd have to omit as.
links for reference: https://www.english-corpora.org/coca/