On the discord I see this under frequently asked questions:
EHG_Kain: Online characters cannot be transferred to offline as the save structure is completely different.
It would be trivial to create an offline save file from the online character, even if the save structure is completely different, any dev worth their salt could write something to take the data from the online save structure and create the offline save file from it.
I personally only play online and only during a cycle/ladder/season, but am making this post because If the question is “can online characters be transfered to offline” then the answer is obviously yes, unlike what the FAQ says. Can and will are different. If you won’t, that’s fine, but the response says you can’t and that’s just not true.