Hi marcpurcell,
There are no options in After Effects to handle a complete PSD [multi-] layered file with several channels (e.g., Buffers). It is something I have requested from Adobe over a decade ago, many times, as many other people, I’m certain about this. As an result of this problem, the Render-Settings> Save> Compositing Project File, ones enabled un-checks the Multi-Layer-File options in the Save panel.
You can import a Photoshop File/Sequence a Composition (Import Window>Options). But again, it allows for one alfa channel, and since many Multi-Passes are often in the channels, this is a no go. No healing of the file to my knowledge in Ae.
https://helpx.adobe.com/after-effects/using/importing-interpreting-footage-items.html
I went through this tutorial and found no mentioning of this, sorry about this. I had to explore Ae CC 2017 for this again, if there is finally an option to split channels off the file. I couldn’t find anything. (Using Ae since 1996, I’m not clear why Adobe is not fully supporting its own main image format [PSD] in layers AND channels.) I would use the Multi-Pass setting (vs Regular Image) to have all files in one format. Add RGBA to the Multi-Pass, then)
You have to re-render, sorry. I would use the Render-Settings> Save> Compositing Project File option (It needs an Plug in for that in Ae, let me know if you haven’t already installed this.) This creates an “.aec” file and this file [after effects compositing] file is like a sub-projectfile, which contains a lot of information, to import all the passes and more you will render. One file dragged into the Project Pane and all files are already imported.
https://help.maxon.net/us/#50775
https://help.maxon.net/us/#DRENDERSETTINGS-RDATA_GROUP_SAVE
Let me know if there is anything else, I’m happy to look into it.
All the best