Update: Yesterday's SC6 patch (released 2020/08/03) updated the save format. This made the previous version of sc6save tool incompatible with exporting and importing custom characters. I've updated the tool so it works with the game again. If you have a version lower than v1.4 of the tool, you should update it.
This tool lets you decrypt and re-encrypt save files from SC6 but its main usage is for letting you export and import custom characters embedded into PNG image files.
The above video shows how to use the tool.
Direct download link to save tool: http://www.fluffyquack.com/tools/sc6save.rar
More info about the save tool including source code link: https://steamcommunity.com/sharedfiles/filedetails/?id=1619160956
And here's a handy website for downloading and sharing custom characters compatible with the save tool: https://cascards.fluffyquack.com/post/list