It is possible to register add-ins in XLL format by listing them as companion files to your workbook.

To do this, you need to have 2 versions of your add-in: 32-bit and 64-bit formats. The filename’s extensions must be modified as shown on the screenshot below, so that XLS Padlock registers the correct version with Excel at runtime:

Then, to load the add-in in your workbook, you can use the following VBA code:

Public Function PathToCompiledFile(Filename As String)
Dim XLSPadlock As Object
On Error GoTo Err
Set XLSPadlock = Application.COMAddIns("GXLSForm.GXLSFormula").Object
PathToCompiledFile = XLSPadlock.PLEvalVar("XLSPath") & Filename
Exit Function
PathToCompiledFile = ""
End Function
Private Sub Workbook_Open()
    Dim success As Boolean 
    ' Load XLL
    success = Application.RegisterXLL(PathToCompiledFile("sample.xll"))
End Sub