You don't. Don't stuff stuff into your book!
Just because you have a vague idea, what could be wrong, does not mean that it is the culprit. Or that there is a culprit at all. Maybe your amount of dialogue is just fine. How many test readers have you asked?
Adding stuff just because something feels slim is almost always a bad idea. Try to handle your (imaginary?) lack by practicing. Start a short story without using dialogues at all. Skip even the most obvious. If you think, you really have to add a dialogue skip that, too.
After you have managed this struggle, reread your work. You will find some nice narration and most probably a terrible, terrible lack of dialogues. Because you've listened to some weird guy on the Internet telling you, you should leave them out.
Mark the sections where dialogues are missing very badly. Count the length of these sections.
Now add in the dialogues (yeah, you are allowed to add stuff now ;). After you have finished, reread. Change it till you are happy with the result. Now count the length of the dialogues. Compare it with the dialogue-free part. You have a rough estimate, what the ratio for your writing could be.
Sleep one or two nights on it and then pick up your 40,000 words book again. Reread and look if really something is missing.
Still unsure? Repeat. (Oh, yes, you could also ask your test readers ...)