They announced official support for porting/using iOS and Android apps to Windows 10 a while back, but ever since that announcement (where they mentioned how Candy Crush was ported very easily from iOS), we haven't really heard anything. Hopefully that changes in the ramp-up to launch of W10 Mobile.
You can install APKs using ADB in the preview versions. ADB sees the Windows Phone device as a valid Android device. Seems most apps that can be sideloaded into Amazon devices (aka: don't depend too heavily on Google Android Services) work.