Yes - but only until the end of the phase :
21.B.d Leaving a Combined Unit
(...) any game effects that would affect the Combined Unit (such as Banner Enchantments) remain in effect for all parts of the Combined Unit until the end of the phase (even if a Character leaves the unit), unless specifically stated otherwise (e.g. One Turn spells).
But is it possible to give only fortitude 5+ to the character, even inside of a unit ? Or rather saying that you give it to the characters, extented to the unit, and then leave the character out of the unit so that the Character keep the fortitude and the unit will just have it until the end of the phase ? :p
I like Stalkers.