But there was no function to migrate or import existing mails into the newly created email account.

So I have searched the Internet and found mb2md.pl . Mbox to Maildir migration perl utility. You can download and save it into PATH registered folder and rename it to mb2md and chmod 755 mb2md to execute it.

Inbox import : Importing source_mbox_file into current maildir account. Note that /home/userid/target_dir is the root folder of your maildir account. You will need to use full path for both source file and target dir.

mb2md -s  /home/userid/source_mbox_file -d /home/userid/target_dir

Multi files in a folder : multi mbox files are located in /home/userid/source_dir and the /home/userid/target_dir is the root of your maildir account. Also you need to use full path for source and target dir.

mb2md -s /home/userid/source_dir -R -d /home/userid/target_dir

After importing mutliple mboxes , you want to login to your Squirrelmail or Roundcube webmail and subscribe mailboxes to make them showing in the mailbox list.