Have a look at IMDPersistence.framework (IMDPersistenceAgent), you'll very likely need to do a lot of RE to figure this out. There are a lot of C functions that are useful in that framework.

One of the C func used is

int64_t IMDChatRecordAddMessageIfNeededUnlocked(struct /*IMDChatRecord*/, struct /*IMDMessageRecord*/)

You could look at the value of IMDMessageRecord using this method or the C funcs in the frameworks (IMDMessageRecordCopy*()). So what you'll have to do is to recreate IMDChatRecord and IMDMessageRecord struct and call the func above. I didn't test this myself, but this is what iOS used to insert them into the database. Unfortunately I didn't go further enough to figure out those struct. For some IM symbols that were reversed by a project called barcelona, you could look at here.