the processors is supports is the i3-2120, i5-2320, i5-2500, and i7-2600. The i7-2600k just means that the cpu is unlocked and can be overclocked. The bios of the Lenovo most likely will not allow it to overclock.
The socket is the same for all the cpus listed so the 2600k will work in the motherboard, just watch the temps if your moving from a i3-2120 to an i7-2600k as the heatsink inside might not be able to handle it.