Please add the ability to pin AppImage to Dash.
I guess it depends on Apps, I am using Telegram as AppImage and I can pin it everywhere I want, But you can try to put your app into /Home/[USER]/.local/share/applications folder, if it shows in the Menu list, you can add it to the panel
I use 2 AppImages and they also can be pinned to the dash, but I don't know if they always could be pinned or if it's because I added their .desktop files to ~/.local/share/applications (the AppImages themselves I have in ~/AppImage).
There are also some apps that will manage your AppImages for you like that so you don't have to do it manually.