Is it possible to enable "double tap to wake" on android devices which do not support it?
On some devices yes... but it requires kernel support, meaning you need a custom kernel. There are several known methods of enabling it with a custom kernel and it depends on the screen type, SoC, and other factors, but in most cases it causes an additional 2-3% battery drain per hour to support DT2W or S2W because the digitizer needs to be powered all the time and the device wasn't designed for that. 

Actually implementing it is more of a development question than an Android question though, it requires manipulation of the kernel code and building a new boot image and flashing it.