The StarWalker fountain pen leaves the factory as a cartridge pen. A piston converter can be retrofitted to some of the versions, though not Montblanc's own converter: that one is too thick for the StarWalker's slim barrel. Individual converters from other makers with a smaller diameter will go in, and only trial and error will establish which converter fits which version. On some StarWalker versions the design deliberately rules out fitting a converter at all.
In everyday use there is a good deal to be said for the cartridge in any case. Filling a converter means dipping the pen into the ink past the section, and the StarWalker's fluted section picks up ink that lodges in the grooves and is hard to remove completely; the result is stained fingers while writing. Anyone unwilling to give up bottled ink does better to refill empty cartridges with a syringe, which avoids dipping the section altogether.