Present fluid based 3D models for describing runaway electrons (REs) (e.g., in M3D-C1 and JOREK) capture the back-reaction of the REs onto the MHD modes, but do not resolve the RE phase space dynamics or the transport in stochastic fields accurately. The work presented here contributes to building a hybrid model where REs are described by a full-f relativistic particle in cell (PiC) approach while a fluid model is used for the background plasma. Concretely, the 3D kinetic avalanche source is implemented into JOREK[^1] via a relativistic collision operator and a periodic resampling of the markers to constrain computational costs while the number of REs grows exponentially. The implementation is benchmarked against analytical expressions from literature[^2] and good agreement is found when applying the same cut-off momentum for the large-angle collisions. The implementation is general enough for 2D and 3D applications and fully kinetic as well as gyro-orbit particles already. An outlook is given to first applications, which will focus in particular on the competition between the avalanching source and stochastic field losses and be complemented by the implementation of nuclear sources. [^1]: M. Hoelzl, G. T. Huijsmans, S. J. Pamela et al., Nuclear Fusion, vol. 61, no. 6, 2021 [^2]: P. Aleynikov and B. N. Breizman, Physical Review Letters, vol. 114, no. 15, 2015