Updating PSD's in AE

If you imported as a composition then the only layers in that composition are the original layers. Any new layers you add in Photoshop will not end up in the original composition. 

There is a clunky workaround. Actually, there are a couple of clunky workarounds but here's the one that I use.

• When you decide you need a new layer in a comp created from a Photoshop file select the layer below where you need the new layer and press Ctrl/Cmnd + e to edit the original PSD file
• Add your new layer or new layers and save the PSD making sure that you accurately name the new layers
• Return to After Effects and create a new folder in the Project panel, name it Temp and import the PSD  as a comp again retaining layer size if that is the way you created the new comp
• Open the new composition
• Select the new layers you have created and copy them
• Paste them in the original composition
• Return to the Proejct Panel and select the layers you have just created and imported into the new comp and drag them to the origiaal folder AE created the first time you created a comp from the original PSD
• Dlete the Temp folder you created when you imported the modified PSD file

Ctrl/Cmnd + e will still open Photoshop to edit the new layer. No files will be missing. Everything should work just fine.

Another option would be to add your new layer, then import the PSD again but this time just pick the new layer and drag it to the timeline. If you choose layer size you will have to position the layer, but if you choose document size it will be in the right position and you'll have to adjust the anchor point. Then you can rename the imported layer to match the layer name and just drag it to the timeline.

Most of the time that this problem shows up people are trying to use AE as a video editor and the comps are longer than a few seconds. If that is the case consider breaking up the comps into shorter segments and keeping them to a single sentence or phrase of audio. 

I don't think that's possible, a small trick you can always keep a few empty layers in your PSD file to use them later if you want and another or import the new layer from a new PSD file.