Looking at the Mellotron V .mrk assets, the data indicates that .mrk is a variation on PKZip compression. The first two “magic number” bytes (504B) are hex for “PK”; note also the recurring PKZip-style file headers and master directory at the end of the file.
So … there should be a way to get a set of 35 custom samples into Mellotron V. Eight user samples in a patch? Rookie numbers. Don’t make a Mellotron and then make it difficult to use. Don’t follow in IK Multimedia’s footsteps. Give us thirty-five samples, with the full complement of parameters embedded in the factory sample sets.