Judging by CPU support for some Z270 boards (for example, https://www.asus.com/us/Motherboards/PRIME-Z270-A/HelpDesk_CPU/), yes you should be able to use Skylake CPUs on Kaby Lake boards.
Judging by CPU support for some Z270 boards (for example, https://www.asus.com/us/Motherboards/PRIME-Z270-A/HelpDesk_CPU/), yes you should be able to use Skylake CPUs on Kaby Lake boards.
When you say upgrade path, do you mean you're planning on getting an i3-6100 with a 100-series mobo now, and then upgrading to a Z270 mobo later? Because while that would work, it would be a waste of money.