How to add your own rand_poll implementation in openssl?

I'm working on porting openssl library to uwp(Universal Windows 10 Platform). The standard rand_poll implementation doesn't seem to work for uwp. I'm thinking adding a customized implementation. For this, i have created a .cpp file(the support for namespace is not present in C, so i had to add a cpp file), which contains my customized implementation of rand_poll(). How do i add this file to the openssl build system so as to use rand_poll functionality from my file rather than from rand_win.c?

Any pointers and suggestions are greatly appreciated.