You can import PNG images as static screens into a project from the Mac app. To do this, go into the project that you want to import the images and select “Edit > Import PNGs…” option from the desktop menu up top.
You can select any PNG image and click “Import” button from the revealed menu. Selected PNGs will be uploaded to your project as static images without any layers details.
☝️ Since this is an image not exported via the plugin, it will not contain any technical resources like layers, assets and code snippets for screens imported as PNGs. To export the screens with their technical resources, you need to export them directly from the design tools through the plugins.