[ravagernl]

local function createBottomArt(skinnr, bottomnr)
  local bgFile = "Interface\\AddOns\\!GrimUI\\Art\\Skin"..skinnr.."\\Bottom"..bottomnr
end

Something like that?

or maybe you are looking for something like this:

local function createBottomArt(skinnr, numparts)
  for i = 1, numparts do
    local bgFile = "Interface\\AddOns\\!GrimUI\\Art\\Skin"..skinnr.."\\Bottom"..i
  end
end