You can do anything, but is it a good idea.
If you put that flat over an existing sheened paint, without a good scuff sanding it wouldn't adhere well. It would get dirty real fast because flats are very porous. It would not wash at all. And it looks wrong.
IMO, the answer is no. Just because you have it, doesn't make using the wrong product a good idea.
Use a satin to semi that is an enamel- made for the job. Many good ones to choose from. Most of them are not found at home depot...