There is no way or tool to convert After Effects to work with Shotstack. You can certainly use those templates as inspiration and there might be elements in them that you can export as an mp4 file which will work with Shotstack. Then you could overlay text or other dynamic elements like images or other video clips.
As an example this template, the background animation of the Christmas tree was made using After Effects but the text is added using Shotstack: Christmas personalised message video template — Shotstack
And in this guide, it shows you how to create an overlay animation in After Effects, export it as a QuickTime MOV file and use it as an overlay transition effect between two clips: Create an overlay transition using After Effects — Shotstack
There are ways the two can work together but no direct 1:1 conversion. If you need more assistance we can put you in touch with designers who might be able to take a template as inspiration and make something similar.