The list importer allows you to import a text file containing a list of still images and have it be treated as a single animated layer.
To use it, simply create a text file with the filename of each still frame listed per-line. Save this text file with the extension
LST in lowercase). Import it into Synfig like you would any other image and you are done!
The default framerate is 15 frames per second. If you would like to use a different frame-rate, add a line to the file like this:
Now the frame-rate will be 24 frames per second.