The thing with going in low tuning is that you have to have the right string gauge for it to get the note played right which is why bass strings for example are much bigger than guitar string: they are designed to produce lower notes. But going beyond that, the instrument is also designed to be able to take such string gauge so in theory, you could take a pack of 8 string and just cut out the 2 higher string and be left with just the lower ones but first problem you could encounter would be:
- First, depending on your guitar, the nut wouldn't be cut to accommodate for larger strings
- Second, depending on the instrument, too much tension could produce damages to a 6 string guitar since they weren't built with that high string tension in mind. You're lowkey putting some bass strings on it and it's not gonna like it.
Then you could take a relative heavy gauge 6 string guitar pack and tune it down to 8 string tuning but I think you'll find the string tension very ''wobbly'' and not very comfortable to play with.
In conclusion, you can experiment with that but these would be the main consideration I would give you before you do anything!
Good luck!