See also Configure Neomutt on mobile. 
There is one step more; Deltachat mails may edit previous messages. Also, many statements in a chat make zero sense taken out of context. I think you’d need to resend all these mails to a Deltachat client to read them easily as a human. Or write your own script to reconstruct the chat from the mails.
Compared to, say, Pidgin, which lets you export chats as HTML or plain text, this is complicated.
…A Pidgin plugin for Deltachat? 
I also suggest that Deltachat could use a standard mail storage format..