You're in luck.
Quick Cookie Manager does exactly what you want.
The trick is to export cookies from a container, and then ensure the same container exists before performing the import operation.
Because the exported cookies are stored in a human-readable format, you can even edit the export file by hand to adjust to which container the cookies will be imported.
This extension is fully compatible with the Multi Account Containers and Temporary Containers extensions.
This Firefox extension is a rare Mozilla Recommended extension, which means Mozilla ensures it meets "the highest standards in security, functionality, and overall user experience".
It is available from AMO here:
https://addons.mozilla.org/firefox/addon/cookie-quick-manager/
And on GitHub here:
https://github.com/ysard/cookie-quick-manager