I've been trying to use "Add Image" in the edit mode of Acrobat Pro DC to add figures (as pdf files) to an existing pdf. But although I can see the list of these pdf figure files, they are not highlighted, so I can't click on them, or open them. I have the latest updated version of Acrobat Pro DC, runing in macOS 12.3.1 (Monterey).
You can create a custom stamp from a pdf, another option is to place your original pdf into InDesign and then place the pdf figures on top and export to a new pdf.
As mentioned that you are not able to insert images that are in a PDF format, right? If that is the case, then convert them to image using the export PDF tool and select the image format you want. If not, Would you mind sharing a few more details to understand the issue better?
Looking forward to hearing back from you.
The thing is, a PDF isn't actually an image. So the menu is right not to offer it.