The short answer is that native speakers use the verb disguise transitively, that is, with a direct object. So one speaks of disguising oneself.
This is true, it seems, even though your sentence has two meanings:
To get into the building I'll disguise myself as a reporter.
To get into the building I'll put on the clothes of a reporter.
But it can also mean
To get into the building I'll disguise myself as a reporter (does)
To get into the building I'll disguise myself using the same method as a reporter disguises himself
In other words, since reporters are not allowed in the building, reporters have to disguise themselves as something else in order to get in.
And since I'm a clergyman and clergymen are also not allowed in the building, I will disguise myself the same way as a reporter disguises himself.
Note this does not mean you have to disguise yourself as the same thing that the reporter disguises himself as. The reporter may disguise himself as a farmer, you might disguise yourself as a fireman.
A parallel usage of what I'm taking about is
I'll play tennis as a tennis pro (does)
I will play tennis in the same way that a tennis pro plays tennis.