Considering upon Jolla's declarations that Sailfish OS is be able to use software from following platforms

Sailfish (natively created + ported like from Qt, Symbian, MeeGo - developers have reported that porting a Qt written software with Sailfish SDK takes a few hours only)
MeeGo (because of backward compatibility thanks to MeeGo code legacy included in the Mer core)
Android (using built-in Alien Dalvik by Myriad Group, which will allow to use it like a native, but with limitations which can happen about UI, if not ported and adjusted)
Unix and Linux (as Sailfish is Linux then using such a software is possible, especially RPM packages, either in terminal/console mode or with limitations implying from using Sailfish UI, if not ported and adjusted)
HTML5 (because it is ready for this kind of software then use oncoming FireFox OS software can also be possible probably)