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You decide you want to do it despite what the spell says, then you write up stats for a weapon that has it, then publish a book with that weapon in it.

The point of the note in the Speed Weapon spell is that you can't cast that spell onto an existing magic weapon. The Battle Fury Blade isn't magical until after the TW process which includes casting Speed Weapon into it. After all, the BFB doesn't actually have the SW spell cast on it, it has very specific abilities that are a culmination of all the required creation spells.

(ignore the post above mine)