I have used the work done by seba to provide vbox on DSM 6.2 with the 4.4.0 kernel.
I have compiled it for apollolake only so far and it works.
https://github.com/dan-pure/virtualbox4dsm/releases/download/6.0.8-130520.1-k4.4.0/virtualbox4dsm-apollolake-6.0.8-130520.spk