From PSD to PSB

If you use File > Place Embedded... to place a PNG file, then Photoshop places it as a Smart Object. When you double-click on the Smart Object, then it will open that placed PNG.

If you want the contents of the Smart Object to be a PSB, then you could do the following:

1. Double-click on the Smart Object in the document to open the PNG file
2. Use File > Save As... to save a PSB version of the PNG to a location where you can find it
3. Go back to the main document
4. In the Layers panel on the Smart Object layer (and to the right of the thumnail) right-click and in the contextual menu that appears, select Replace Contents...
5. Navigate to the PSB file that you saved