yes, you can do that, but it takes several steps. Here is a suggested approach:

1. first, install the addon named importexporttools.
2. assuming you want to merge folderB and folderC into folderA, here are suggested steps:
   • highlight folderB, rightclick and select importexporttools option to export eml and do that to a non-thunderbird folder (folderX)
   • repeat the process for folderC
   • you now have messages from folders B and C in external folderX in Windows
   • now, click folderA and rightclick and select importexport option to import eml and locate and select the Windows folderX.
   • you now have all messages in folderA. After verifying, you may now delete folders B, C, and X.

Thank you I'll try it and let you know.