yes with the new asrock skylake oc bios that came out today, you can overclock non k skylake processors on asrock z motherboard. other companies will soon come out with their bios mods as well.
with a 120mhz base clock and a little bit more vcore the 6400 can hit 4.0ghz not including its turbo bins which will go as high as 4.4ghz with a 120mhz bclk. the 6600 has a base multiplier 1 higher than the 6500 but has a 2x higher turbo bin on the single core. it is a marginal increase in speed.
you will never get a "huge" overclock on a locked i5 anyways. and many people use the hyper 212 evo to put their 2600k, 3770k, 4790k, 6700k to a modest 4.4ghz. 4.0ghz would be easy for a 6500+hyper212evo.