[Phanx]

Before asking for help/feedback on your code, your first step should always be to load it in-game, or at least run it through a Lua syntax checker, and attempting to fix the basic syntax errors. In this case, "if for ... end else ... end" is not valid Lua syntax.

If the TimerTrackerN bars have the same structure as the MirrorTimerN bars, then you can just add them to the existing list of bars the "for ... end" loop should iterate over (on line #3):

-- mirror castbar!
  lib.gen_mirrorcb = function(f)
    for _, bar in pairs({'MirrorTimer1','MirrorTimer2','MirrorTimer3','TimerTrackerTimer1','TimerTrackerTimer2','TimerTrackerTimer3'}) do   
      for i, region in pairs({_G[bar]:GetRegions()}) do
        if (region.GetTexture and region:GetTexture() == 'SolidTexture') then
          region:Hide()
        end
      end
      _G[bar..'Border']:Hide()
      _G[bar]:SetParent(UIParent)
      _G[bar]:SetScale(1)
      _G[bar]:SetHeight(16)
      _G[bar]:SetWidth(280)
      _G[bar]:SetBackdropColor(.1,.1,.1)
      _G[bar..'Background'] = _G[bar]:CreateTexture(bar..'Background', 'BACKGROUND', _G[bar])
      _G[bar..'Background']:SetTexture(cfg.statusbar_texture)
      _G[bar..'Background']:SetAllPoints(bar)
      _G[bar..'Background']:SetVertexColor(.15,.15,.15,.75)
      _G[bar..'Text']:SetFont(cfg.font, 14, "THINOUTLINE")
      _G[bar..'Text']:ClearAllPoints()
      _G[bar..'Text']:SetPoint('CENTER', MirrorTimer1StatusBar, 0, 1)
      _G[bar..'StatusBar']:SetAllPoints(_G[bar])
      --glowing borders
      local h = CreateFrame("Frame", nil, _G[bar])
      h:SetFrameLevel(0)
      h:SetPoint("TOPLEFT",-5,5)
      h:SetPoint("BOTTOMRIGHT",5,-5)
      lib.gen_backdrop(h)
    end
  end