Solved! For anyone else looking for something similar, I found this code works:
Dim objPresentation As Presentation
Dim objPowerPoint As New PowerPoint.Application
Dim intobj As Integer


Set objPresentation = objPowerPoint.Presentations.Open(FileName:=(rs.Fields("Extra slides 1")))

For intobj = objPresentation.Slides.Count To 1 Step -1    
objPresentation.Slides.Item(intobj).Copy
    objPowerPoint.Presentations.Item(1).Slides.Paste 1
    objPowerPoint.Presentations.Item(1).Slides.Item(1).Design = _
        objPresentation.Slides.Item(intobj).Design
    
Next intobj
objPresentation.Close