Others have done this; I have not, but I documented it :). My problems with Neomutt were internal to Neomutt, nothing to do with DC.
DC is standards-based. Any Autocrypt-compatible mailclient should work. Chatmail servers only allow Autocrypted mails, but they should take any Autocrypt mails.
It is possible to manually import (and export) keys. I’ve been meaning to write a tutorial on this (EDIT: here, Advanced contact-adding tutorial) because there are many different methods. I will try to get to this this week, but lest it is urgent I will summarize a likely method: send an .asc file as an attachment in DC, or send a DC contact (which is an .asc file under the hood).
CORRECTION: By “.asc” obviously I meant “.vcard”. Apologies, I need to catch up on sleep! See next post for much better instructions.
They should send the first message encrypted, with an Autocrypt header. An unencrypted mail sent to a Chatmail account will bounce. They need to obtain the recipient’s public key first. If they have a copy or it is pblished they can import it into DC.
Sorry for the partial reply.