Can you make an item that, when crafted, will already have an enchantment?

When editing the item, under "Triggers" there should be a spot that says "When item is crafted/smelted". Add a procedure to it that uses the "Add enchantment with level ___ to (Provided itemstack) type ___" under the item procedures. This should ensure that the item has fire aspect when crafted.

alternatively, you could add a procedure to "When living entity is hit with item" that uses the "set Event/Target entity on fire for __ seconds" procedure to set the hit entities on fire, regardless of the enchantments.