The StarWalker twist ballpoint is designed around Montblanc's own ballpoint refill, which differs in length, diameter and tip shape from the usual standard refills. Suitable alternatives from other manufacturers are not documented; standard refills from the stationer's, from Schneider for instance, will not simply drop in. That a refill out of a promotional ballpoint slides in mechanically does not mean it is intended to. Anyone going to a specialist dealer should take the pen along with its existing refill and have the dimensions compared.
Least suitable of all are rollerball and so-called capless refills. They hold thin, free-flowing ink and dry out within a few days in a body without a cap; that accounts for refills which stop writing after a short time without actually being empty. The ink also runs out as soon as the extended tip meets fabric, leather or anything else. Rollerballs have a cap for precisely this reason, and capless models have a mechanism that prevents the tip being advanced by accident. The differing dimensions of the refills are deliberately chosen so that the systems cannot be interchanged. Anyone wanting to use a different writing system should reach for the appropriate body; the StarWalker is available as a liner and as a capped rollerball as well.