If I were at this impass, I would unravel back to where you have the original stitches put onto the size 8 needles.
At this point I would make sure that I had the live yarn at the point needed to cast on more stitches attached to the stitches on the size 8 needles.
I would then get a zip type bag and place the size 6 needles with the stitches on them in that bag to make sure they are separated.
Then I would take one of my double pointed needles to do the knit cast on being careful to have the finished stitch be on the size 8 circular needle.
Once you have cast on the number of stitches needed, place marker and join as directed - making sure that you have a circle without a twist.
Now you are ready to knit the cap portion with the joined stitches using the knit stitch which will magically create stockinet when knit in the round.