Responding here and on the Steam forums:
You can do it by having the Fire Up set to Move Up's key and do that for all the fire directions (Up down left right). You can then map more alternatives if you need/want to.
KeyBindingV3:None|None|None|None|None|None|None
KeyBindingV3:TakeScreenshot|F12|None|None|None|None|None
KeyBindingV3:MarkAllLocalMessagesAsNew|BackQuote|None|None|None|None|None
KeyBindingV3:Debug|F3|None|None|None|None|None
KeyBindingV3:DebugNetwork|F2|None|None|None|None|None
KeyBindingV3:StartStopPerFrameDebugger|F10|None|None|None|None|None
KeyBindingV3:NextFrameInPerFrameDebugger|F11|None|None|None|None|None
KeyBindingV3:Ingame_MoveUp_UKB1|W|UpArrow|None|None|None|LeftStickUp
KeyBindingV3:Ingame_MoveLeft_UKB1|A|LeftArrow|None|None|None|LeftStickLeft
KeyBindingV3:Ingame_MoveRight_UKB1|D|RightArrow|None|None|None|LeftStickRight
KeyBindingV3:Ingame_MoveDown_UKB1|S|DownArrow|None|None|None|LeftStickDown
KeyBindingV3:Ingame_AltUp_UKB1|None|None|None|None|None|DPadUp
KeyBindingV3:Ingame_AltLeft_UKB1|None|None|None|None|None|DPadLeft
KeyBindingV3:Ingame_AltRight_UKB1|None|None|None|None|None|DPadRight
KeyBindingV3:Ingame_AltDown_UKB1|None|None|None|None|None|DPadDown
KeyBindingV3:Ingame_ConfirmOrAction_UKB1|Return|KeypadEnter|None|None|None|Action1
KeyBindingV3:Cancel|Escape|None|None|None|Mouse1|Action2
KeyBindingV3:OpenChatPopup_UKB4|T|None|None|None|None|None
KeyBindingV3:DebugReloadLanguageFiles|F5|None|LeftControl|None|None|None
KeyBindingV3:DebugReloadImageFiles|F6|None|LeftControl|None|None|None
KeyBindingV3:OpenDebugMenu|L|None|None|None|None|None
KeyBindingV3:HoldToChangeLeftClickIntoMiddleClick_UKB1|LeftControl|None|None|None|None|None
KeyBindingV3:DebugDoMemoryProfilingSample|F13|None|None|None|None|None
KeyBindingV3:DebugReloadXMLFiles|F4|None|LeftControl|None|None|None
KeyBindingV3:DebugClearGraphicalEffects|None|None|None|None|None|None
KeyBindingV3:Debug_IncrementKerningX|Equals|None|None|None|None|None
KeyBindingV3:Debug_IncrementKerningY|Equals|None|LeftShift|None|None|None
KeyBindingV3:Debug_DecrementKerningX|Minus|None|None|None|None|None
KeyBindingV3:Debug_DecrementKerningY|Minus|None|LeftShift|None|None|None
KeyBindingV3:Debug_IncrementKerningMode_UKB1|RightBracket|None|None|None|None|None
KeyBindingV3:Debug_DecrementKerningMode_UKB1|LeftBracket|None|None|None|None|None
KeyBindingV3:Debug_DumpKerning|Backslash|None|LeftControl|None|None|None
KeyBindingV3:Debug_DumpIndirectSpriteBatches|Delete|None|LeftControl|None|None|None
KeyBindingV3:Debug_ToggleContinuousKerning_UKB1|Backslash|None|LeftAlt|None|None|None
KeyBindingV3:ToggleGUI|None|None|None|None|None|None
KeyBindingV3:SwapLeftAndRightMouseButtons|None|None|None|None|None|None
KeyBindingV3:OpenMenu_UKB3|Escape|None|None|None|None|Menu
KeyBindingV3:ToggleDebugRanges|F7|None|None|None|None|None
KeyBindingV3:InteractOrPickUpItem_UKB2|E|None|None|None|None|LeftBumper
KeyBindingV3:Sprint_UKB3|LeftShift|None|None|None|None|LeftTrigger
KeyBindingV3:WalkSlow_UKB3|LeftControl|C|None|None|None|LeftStickButton
KeyBindingV3:Gamepad_FireUp_UKB3|W|None|None|None|None|RightStickUp
KeyBindingV3:Gamepad_FireLeft_UKB3|A|None|None|None|None|RightStickLeft
KeyBindingV3:Gamepad_FireDown_UKB3|S|None|None|None|None|RightStickDown
KeyBindingV3:Gamepad_FireRight_UKB3|D|None|None|None|None|RightStickRight
KeyBindingV3:Gamepad_FireUpAlt|None|None|None|None|None|Action4
KeyBindingV3:Gamepad_FireLeftAlt|None|None|None|None|None|Action3
KeyBindingV3:Gamepad_FireDownAlt|None|None|None|None|None|Action1
KeyBindingV3:Gamepad_FireRightAlt|None|None|None|None|None|Action2
KeyBindingV3:UseEnergySystem|None|None|None|None|Mouse1|RightTrigger
KeyBindingV3:UseMissileLauncher|Space|None|None|None|None|RightBumper
KeyBindingV3:UseConsumableSystem_UKB4|F|None|None|None|None|Select
KeyBindingV3:ExecuteLastDevScript|F8|None|LeftControl|None|None|None
KeyBindingV3:DebugBatchRefreshTestChamber|K|None|None|None|None|None
KeyBindingV3:FireMainGun|None|None|None|None|Mouse0|None