Since this thread has no reply, then maybe this might be a solution.
Sub CopySlidesToEmbeddedPPTX()
    Dim pptApp As Object 'As PowerPoint.Application 
    ' Late binding
    Dim currentPres As Presentation
    Dim embeddedShape As Shape
    Dim embeddedPres As Object 
    ' As PowerPoint.Presentation 
    ' Late binding
    Dim sourcePres As Presentation
    Dim sourceSlide As Slide
    Dim i As Long
    ' Set references
    Set currentPres = Application.ActivePresentation
    ' **Important:** You need to know the index or name of your embedded PowerPoint object.
    ' You can find this by selecting the embedded object in PowerPoint and checking
    ' its properties in the VBA Immediate window (e.g., ?ActiveWindow.Selection.ShapeRange(1).Name).
    ' Replace "Object 1" with the actual name or index if needed.
    On Error Resume Next
    Set embeddedShape = currentPres.Slides(1).Shapes("Object 1") 
    ' Assuming it's on the first slide and named "Object 1"
    If embeddedShape Is Nothing Then
        Set embeddedShape = currentPres.Slides(1).Shapes(1) 
        ' Try the first shape if name doesn't match
    End If
    On Error GoTo 0
    If embeddedShape Is Nothing Then
        MsgBox "Embedded PowerPoint object not found on the first slide.", vbExclamation
        Exit Sub
    End If
    ' Check if the embedded shape is an OLE object  
    If embeddedShape.Type <> msoOLEObject Then
        MsgBox "Selected shape is not an OLE object.", vbExclamation
        Exit Sub
    End If
    ' Get the embedded PowerPoint Application object (late binding)
    On Error Resume Next
    Set pptApp = GetObject(, "PowerPoint.Application")  
    If Err.Number <> 0 Then
        Set pptApp = CreateObject("PowerPoint.Application")
    End If
    On Error GoTo 0
    ' **Specify the full path to your .pptm file**
    Dim sourceFilePath As String
    sourceFilePath = "C:\Path\To\Your\SourcePresentation.pptm" ' **<-- CHANGE THIS**
    On Error Resume Next
    Set sourcePres = pptApp.Presentations.Open(sourceFilePath, msoFalse, msoFalse, msoFalse) 
    ' Read-only, not visible
    On Error GoTo 0
    If sourcePres Is Nothing Then
        MsgBox "Could not open the source presentation: " & sourceFilePath, vbCritical
        If pptApp.Presentations.Count = 0 Then
            pptApp.Quit 
            ' Quit PowerPoint if it was newly created and no presentations are open
        End If
        Exit Sub
    End If
    ' Activate the embedded object to get its Presentation 
    object  embeddedShape.OLEFormat.DoVerb ppVerbOpen 
    ' Or ppVerbPrimary
    ' Wait a short time for the embedded object to activate (optional but can be helpful)
    Application.Wait (Now + TimeValue("0:00:01"))
    ' Get the Presentation object of the embedded object
    On Error Resume Next
    Set embeddedPres = embeddedShape.OLEFormat.Object
    On Error GoTo 0
    If embeddedPres Is Nothing Then
        MsgBox "Could not access the embedded PowerPoint presentation.", vbCritical
        sourcePres.Close
        If pptApp.Presentations.Count = 0 Then
            pptApp.Quit
        End If
        Exit Sub
    End If
    ' Copy slides from the source to the embedded presentation
    For i = 1 To sourcePres.Slides.Count
        sourcePres.Slides(i).Copy
        embeddedPres.Slides.Paste
    Next i
    ' Clean up  sourcePres.Close
    ' Optionally, you can deactivate the embedded object
    embeddedShape.OLEFormat.DoVerb 
    ppVerbHide
    MsgBox "Slides copied successfully to the embedded PowerPoint object.", vbInformation
    ' Quit the PowerPoint application if it was newly created and no other presentations are open
    If pptApp.Presentations.Count = 1 And currentPres.Application Is Not pptApp Then
        pptApp.Quit
    End If
    ' Release object variables
    Set pptApp = Nothing
    Set currentPres = Nothing
    Set embeddedShape = Nothing
    Set embeddedPres = Nothing
    Set sourcePres = Nothing
    Set sourceSlide = Nothing
End Sub