You can edit the original on any media you import into After Effects by going to the Edit>Edit original of using the keyboard shortcut Cmnd/Ctrl + E
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However, if you imported the Illustrator file as a composition, you're limited to modifying the layers that already exist. You can't add additional ones.
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You can right click on your illustrator layer in you AE timeline and go to "create vector shapes from layer". This will create separate shape layers and allow you to modify each one separately in AE. For me this works many times if I just want to make some small tweaks to the shapes.
Oh, thank you! This is what I wanted to find.