Twilight Arbor armor can be picked with one of the following three prefixes: Rampager's, Rabid or Magi's. But I'd like the Berserker's prefix on the armor pieces. Is it possible to have this specific armor with the Berserker's prefixes?
Unfortunately, no. I've had the same problem. However, most of these armor options are meant to be used for transmuting the skins anyway (i.e. buy the item to be used as a skin, buy a transmutation crystal off the item store and merge the item with a crafted/bought/dropped one with correct stats).
However: According to recent datamining of the client files it seems like ArenaNet might introduce special itemshop items allowing you to reconfigure/change prefixes on items. Although it's yet to be seen how much they'll cost, whether they'll indeed appear and whether they'll allow you to reconfigure such "set" items as well, considering there probably aren't other versions (like Berserker) right now.