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.
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.
Let me know if there is anything else, I’m happy to look into it.
All the best