Phase engineering has extensively been used to achieve metallization of two-dimensional (2D) semiconducting materials, as it should boost their catalytic properties or improve electrical contacts. In contrast, here we demonstrate compositional phase change by incorporation of excess metals into the crystal structure. We demonstrate post-synthesis restructuring of the semiconducting MoTe2 or MoSe2 host material by unexpected easy incorporation of excess Mo into their crystal planes, which causes local metallization. The amount of excess Mo can reach values as high as 10% in MoTe2 thus creating a significantly altered material compared to its parent structure. The incorporation mechanism is explained by density functional theory in terms of the energy difference of Mo atoms incorporated in the line phases as compared to Mo ad-clusters. Angle resolved photoemission spectroscopy reveals that the incorporated excess Mo induces band gap states up to the Fermi level causing its pinning at these electronic states. The incorporation of excess transition metals in MoTe2 and MoSe2 is not limited to molybdenum, but other transition metals can also diffuse into the lattice, as demonstrated experimentally by Ti deposition. The mechanism of incorporation of transition metals in MoSe2 and MoTe2 is revealed, which should help to address the challenges in synthesizing defect-free single layer materials by, for example, molecular beam epitaxy. The easy incorporation of metal atoms into the crystal also indicates that the previously assumed picture of a sharp metal/2D-material interface may not be correct, and at least for MoSe2 and MoTe2, in-diffusion of metals from metal-contacts into the 2D material has to be considered. Most importantly though, the process of incorporation of transition metals with high concentrations into pristine 2D transition-metal dichalcogenides enables a pathway for their post-synthesis modifications and adding functionalities.