Can i charge 18650 single cell using buck converter?
It should work, if the settings are accurate enough. You can set it to 4.1V for extra safety, you won't lose much capacity. Check voltage and current with a multimeter...

You will need to monitor the voltage to know when the charge is finished though. It is not recommended to let the battery charge at 4.2V for too long (most charge control ICs include a timer and various other safety checks).

From mkeith:
The standard method of charge termination for lithium ion (and lithium polymer) chargers is by current. During the CV stage, if the current drops to some low level, the charger stops charging. The timer would be a backup termination method in case the primary termination condition never occurs. For a single 18650, the charger might terminate at like 50 mA or something. 

Also make sure the cell is within the acceptable temperature range. If this happens indoors, it should be.

This starts to sound like a lot of work.

You can buy a 2-cell 18650 charger for little money these days, so that would be a better and safer solution.