To make this work you'll have to at least move these messages into a MailDir like structure. My Dovecot maildir looks like this for the account ahoy@iswhale.com
./iswhale.com/ahoy/.Drafts/tmp
./iswhale.com/ahoy/.Drafts/new
./iswhale.com/ahoy/.Drafts/cur
./iswhale.com/ahoy/cur
./iswhale.com/ahoy/new
./iswhale.com/ahoy/tmp
If you make a new sub-folder, say 'Archive' it would look like this:
./ahoy/.Archive/tmp
./ahoy/.Archive/new
./ahoy/.Archive/cur
The tmp, new and cur are directories created by Dovecot when you make this new folder in a client (eg: Roundcube). When a new sub-folder of Archive is created the directory structure becomes:
./ahoy/.Archive.Sub-Folder/tmp
./ahoy/.Archive.Sub-Folder/new
./ahoy/.Archive.Sub-Folder/cur
So, you will need to adjust your directory paths to make '2015-01' become something like:
./user/.Archive.2015-01/tmp
./user/.Archive.2015-01/new
./user/.Archive.2015-01/cur
Then, once these directories are created you'll copy the '.eml' files (which I hope are just plain-text email of the type expected by Dovecot) into the target 'new' directory. Connect with your preferred IMAP client and all is well.