Would you ask the nice CTA developers to add PNG sequence import to the next rev, please?
Actually with the popconverter it is already possible. That is not only for greenscreen footage.
You can converted your imagesequencence in iwidget or popvideo moviefiles
http://www.reallusion.com/popvideo/default.aspxiwidget and povideo are native RL Formats, that normaly works in CTA
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liebe Grüße vidi