It's not impossible I doubt all crossbows/bows and arrows are all gone since the bombs went off. I also doubt that all knowledge pertaining to making one disappeared with the bombs as well. If theirs enough demand by the fans maybe they'll include one in one of these DLC's or Fallout 4. I personally always wished the Legion would use a bow and arrow or crossbow seems like it would fit them well.
So bows and arrows/Crossbows are destroyed but assault rifles and heavy weapons are A-okay? Granted, the Gun Runners exist, but some guns (i.e. The BAR) are certainly unique to the PC. I don't understand how the knowledge could disappear, I mean the continuity makes no sense--the PC can repair Miniguns and other weapons with high knowledge of repair, and can even make bullets, but when trying to figure out how to fletch a bow and arrow, then string the bow, its impossible? Come on, if older civilizations could make a bows and arrows, and even crossbows, certainly newer civilizations could figure something out.
Fallout 3 had a mod that brought bows and arrows into the game (unfortunately, upon searching, I could not find said mod by SaidenStorm), plus another separate mod that brought Crossbows, and personally, I would welcome them in. The bows and arrows should only be effective on unarmored targets, unless creating an arrow with other capabilities, and the same goes with the crossbow.