Similar to previous titles, Kaizoku Musou will have downloadable costumes. This was shown in Jump this week in addition to what the first two costumes will be. The first one will be out on March 1st (release day) and will be Luffy dressed as a Samurai. A week later on March 8th Nami will be in a kimono. Both of these are based off an earlier color page from the manga.
No word on prices or additional costumes, though they seem highly likely. Check back for more details are they become available.