I would like to merge another email with the existing email account with Thunderbird

You can copy the messages from one account to another. Is that the question? If POP accounts, you can Edit>select all and right-click and select 'copy to'.. If IMAP, it's trickier, as you have the network interaction to consider. In that case, I suggest creating a filter to do the copy. Do not use MOVE, as if problems surface, you may lose messages.