In guided-wave optics, using gratings to couple between single mode waveguides and single mode fibers and vice versa is well-established. In contrast, the coupling between multimode waveguides is more complex and a much less understood topic, even though multimode coupling is essential for the excitation of guided modes from spatially incoherent sources or for the extraction of spatially incoherent radiation from a guided-wave platform. Here, we present the design for a grating that couples multiple modes of a 2D slab waveguide into a multimode fiber and vice versa and discuss the corresponding challenges. We highlight the importance of matching mode numbers and scattering angles and show that the coupling efficiency can readily drop to low values. We present a rudimentary design that illustrates the key issues by demonstrating the coupling from a multimode fiber into a waveguide slab and back into another fiber, which achieves a total efficiency of -34 dB. By modeling the same geometry, we achieve good agreement, which allows us to explain the physics of the coupler and to suggest improvements. Future options are discussed to improve the coupling elements with a better directivity in order to achieve a maximal coupling efficiency. Our findings can be exploited for improving the multimode light injection into and out of integrated guided-wave optical systems.