Is this "rounding out" a present participle or a gerund? Maybe this is not grammatically clear, so I remember some teacher saying native speakers just say things as they come out from the mouth. I don't think Koreans and native speakers don't think about grammar a lot when speaking or writing. I don't consider grammatical expressions in Korean, just speaking naturally.
13)Rounding out the list of Korean stars who are being cast in Hollywood films are : half-Korean actor Daniel Henney, who appeas in X-Men Origins : Wolverine; Jang Dongkun, who appears in The Laundry Warriors; and Lee Byunghun, who appears in the film version of G.I.Joe...
Participle, it seems. They (the stars mentioned) are rounding out.