The key attraction of graphene in spintronics arises from its high electronic mobility and low intrinsic spin-orbit coupling (SOC), which enable long spin relaxation times. However, the weak SOC limits the ability to control spin currents within graphene. A promising strategy for enhancing the spin functionalities of graphene is to introduce proximity effects with other materials. In this context, molecular compounds show great potential for tuning the spin properties of graphene. Here, a novel fabrication methodology is presented that integrates molecular compounds with graphene-based spintronic nanodevices using stencil hexagonal boron nitride (hBN) masks, allowing us to investigate the resulting spin-related effects. First, our non-destructive fabrication technique, confirmed by micro-Raman spectroscopy, preserves the integrity of the molecular compound. Moreover, by combining experimental Hanle precession with a 3D spin diffusion model, it is demonstrated that fullerene (C₆₀) molecules enhance the spin relaxation time of the graphene. The established fabrication methodology can be further expanded to integrate graphene with exotic molecular compounds, such as photochromic and spin cross-over molecules, enabling the exploration of proximity-induced spin phenomena and paving the way for spin-based multifunctional nanodevices.