Is there a way to send text files to kindle ~without~ connecting to internet

Hello!

So basically I’ve seen ways people are uploading docs and fanfics to their kindle and I would like to do that. However, I’m doing the ol’ download several books from the library and then put the kindle in airplane mode and return the books trick to keep the books for longer.

Which means I can’t connect my kindle to WiFi to download the files as it would auto return my books.