Hi,
It is possible without changing paths, but you will need to change your series. Look at this example:
https://jsfiddle.net/BlackLabel/wrhud9ov/
I was able to change the whole border of Poland to red, and playing with
zIndex property I was able to show only a piece of the red border of Poland that borders with countries
from the same series (dark grey color). So, e. g. if I wanted to color the border between Poland and Germany, I would need to keep Poland and Germany in one series and put all other countries to separate series.
But that is not the end of our problems - notice that when we hover over Poland, border disappears. We could set
plotOptions.series.enableMouseTracking property to false or find other solution that would allow us to use a tooltip and would not affect on a border.
Kind regards,
Rafał