Rooted Android device method:

1) Use your favorite file explorer that support root (example: X-plore File Manager)
2) Go to /system/etc/
3) Edit the file "hosts" adding the following line:
4) Restart Neko Atsume
5) Go to daily password prompt
6) Enter "neko"
7) You should be awarded 999999999 gold fishes and 999999999 silver fishes

You can do it again every day.
If you wan to get back to non-cheat passwords, just delete the added line in hosts file.
For non-rooted users, this could be achieved with an intercepting proxy.

You can also make your own server to reply to API requests (you should then use your own IP in hosts file).
The page must return data in this format:
1,password,silver fish count,gold fish count,today's date,
Page must be located at app/nekoatsume/neko_daily_en.php
This page is to show and get password.
You also must do another page which will be called to check password validity. It must return same kind of values.