I’m sorry, but as an AI text-based model, I don’t have access to specific fonts or their extensions. However, I can provide you with some general information about incorporating Japanese characters into a font.

To create a Japanese extension for the Flunkie font or any other font, you would need to design and include the specific Japanese characters, known as kanji, hiragana, and katakana. These characters are essential for representing the Japanese writing system.

To properly design a Japanese extension, it’s crucial to understand the stroke order and structure of each character, as they have distinct visual characteristics. Japanese characters are typically more complex than Latin characters, requiring attention to detail and adherence to established calligraphic styles.

You may consider consulting a professional font designer or typographer with experience in creating Japanese fonts. They can guide you through the process of designing and integrating the necessary characters into the Flunkie font or assist in creating a new font specifically for Japanese text.

Additionally, it’s important to ensure that the font includes appropriate OpenType features and supports the necessary character sets for proper typesetting of Japanese text. These features may include glyph substitution, contextual alternates, and vertical writing support, among others.

When working with Japanese fonts, it’s also important to consider the font metrics, line spacing, and overall visual harmony to ensure that the Japanese characters blend seamlessly with the existing Latin characters in the Flunkie font.

Keep in mind that creating a Japanese extension for a font requires a deep understanding of the language, typography, and the specific technical requirements for Japanese text display. It’s a complex process that may require the expertise of a professional designer to achieve the desired results.

I recommend reaching out to a qualified font designer or typographer who specializes in Japanese typography to ensure the best outcome for your Japanese extension project.