Is it possible to create folders and move messages directly into the DB?

I've analyzed the DB and I understood how folders are created and messages are indexed: are there any problems to move messages and create folders into the DB?

hMail caches some values in memory also when a client connects the the account you are adding messages to there is no telling what will happen. Basically if you do that we will not support hMail if it breaks, you will be on your own so use it at your own risk.