I can't find the information regarding whether dungeon drops/unique/rare items can be slotted or not in a search, memory is fuzzy on that one.

In most mmorpgs, crafting doesn't seem as appetizing when random world drops, quest rewards, and dungeons yield similar, convenient, or even more powerful items. Crafters get that short end of the stick where the fruits of the their labor will never produce legendary swordren like Excalibiur or Masamune or fabled suits of armor.

And although materia isn't quite like that, a high end high quality crafted item that was five slotted with Materia of the end-user's choice, flowing with memories of veterans who've had many adventures roaming the world as rare and epic to me as a flaming bow made of lalafell skulls that shoots dual katana-wielding moogle ninjas instead of arrows dropped from the imperial Witch King that rides Bahamut while setting Gridania ablaze.

But I'm under the pretense that dungeon drops can't be slotted. That they may be pretty beastin' to begin with, just not tailored to the player's playstyle beastin'.

And if that's the case, I think the crafters should be able to create that Ultimate Sword of Doom.