Based on first-principles total energy calculations within density functional theory, we show that a low dose of Mn on Ge(100) initiates in a novel subsurface growth mode, characterized by easy access to, and strong preference for, interstitial sites located between the two topmost Ge layers. Strikingly, such a "subsurfactant action" is preserved even during epitaxial growth of additional Ge layers, analogous to the well-known phenomenon of surfactant action. In contrast, along the [111] orientation, Mn can easily diffuse into the bulk via interstitial sites. These results are discussed within the context of dopant control in dilute magnetic semiconductors.