I just managed to do that outside Construct, but cannot guarantee if there will not be any problems (have not tested it yet) so create a backup.
1. In project directory there is directory "layouts". You have to copy XML file of desired layout (t is called same as layout), from project you want export from to target project.
2.You have to edit .caproj file of your project. Find "<layout-folder>" and below you will see all existing layouts. Here you have to add new line with new layout, composed same way as other layouts.