Not within Kontakt Player. If you were using the full version of Kontakt, you could tinker under the hood a bit more, but it's not something that most people would want to spend their time doing. We have it on our list to add an 8-person bassline moving forward. When that happens, we'll also update the Sibelius/Finale templates.
Here's a workaround you might want to try - although it's not pretty... Create two bassline staves. On the top staff only input using basses 1 through 4. On the bottom staff, only input basses 3 through 6. Since there will be some overlap (with VDL drums 3 and 4) In Kontakt Player, experiment with shifting the pitch of the top bassline up a step or two. Shift the bottom bassline down a step or two. Or instead, you could keep the top bassline as normal, but then detune the bottom one and only input using a couple lower notes of that bassline. Then, you'd create a third staff (muted playback) where all 8 parts would be notated, which is what you'd hand out to your players. The top two workaround staves could be hidden so they wouldn't print in your score.
Thanks for your patience. While the vast majority of VDL users haven't needed 8 separate basses, we realize it's something that some folks with bigger groups use.