The rapid development in the design and synthesis of covalent organic frameworks (COFs) brings opportunities in tuning their electronic and magnetic properties and expanding their applications. Controlled chemical doping is a traditional route to modulate the charge carrier injection and transport properties in organic molecular and polymeric semiconductors; it represents a natural strategy that, however, has not been explored systematically for COF monolayers (2D COFs), especially when interfaced with inorganic substrates. Here, considering alkali metal (Na) atoms as conventional dopants, we investigate at the Density Functional Theory level the n-type doping of the porphyrin-based COF, COF366-OMe, in the form of both a freestanding monolayer or as interacting with a graphene substrate. The COF monolayer and COF/graphene complex are found to be efficiently n-doped by accepting a full electron from each Na dopant. On the COF/graphene complex, while a Na atom binds more strongly to the COF than to graphene, the transferred electron distributes between them. As a result, the Fermi level of graphene shifts above the Dirac point, whereas the conduction band minimum of the 2D COF strongly stabilizes; the consequence is a marked reduction in the electron injection barrier between the graphene sheet and the 2D COF. Our study highlights the key role that controlled chemical doping of COFs can play in tuning their charge injection and transport properties for optoelectronic applications.