Try hooking GameTooltip_AddNewbieTip?
GameTooltip_AddNewbieTip(XPBAR_LABEL, 1.0, 1.0, 1.0, NEWBIE_TOOLTIP_XPBAR, 1)
Something like:
oldGameTooltip_AddNewbieTip = GameTooltip_AddNewbieTip
GameTooltip_AddNewbieTip = newGameTooltip_AddNewbieTip
function newGameTooltip_AddNewbieTip(v1,v2,v3,v4,v5,v6)
oldGameTooltip_AddNewbieTip(v1,v2,v3,v4,v5,v6)
GameTooltip:AddLine("Another line")
end
There's a way to pack/unpack arguments (the v1..v6 bit) which I've seen used but don't understand it completely.