There is nothing official about there being any members of the Fire Hawks, who were not lost with the chapter.
However, there is no reason for your purposes there could have been some stragglers on duty somewhere else if you wanted to paint your army as the Fire Hawks, or you could always play your army as a historical army, before they Vanished (like around Badab War era or what have you). They wouldn't have access to any Primaris units though.