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Would it be possible to import a jpg and append this as a new frame to the current layer?

Somehow the import footage creates a new layer and I am not quite sure what happens if I open the file using the file/open menu.

Kind regards,
Frank

You can always copy one (or more) frames from one layer and paste it into another.
Or you shift the new layer with the one (former JPG) image to the end of the previouos layer and merge them.