Do the rules allow that and what is the result if yes?

There was a discussion for the other variant where a stack of 5 can become owned when (after?) the colour is claimed, dixit Chris Burn:

https://boardgamegeek.com/filepage/142605/lyngk-rules

Is is the same if the colour on top of the 6-stack is not yet claimed?

The question is the game ends as soon as the colour is claimed or if it can become a hub for a lyngk move is maybe less relevant as it is a win.

Last sentence is : “The question whether the game ends…“.

The only way a stack (with more than one disk) can be neutral is to have an unclaimed color on top.

Based on that thread, I'm strongly of the opinion that you can create a 6 with a neutral on top.  It would remain on the board (unless the other player can still claim a color, in which case they could do so next turn and win).