The 5th edition Emerald Empire book does present the Kitsune Impersonator on page 239 as an option to play a Fox spirit. But it doesn't present any family or clan they belong to or has a rule for picking a family. Don't they choose a specific family or do you need to make one according to the Ronin book?
No, they have a special rule that allows them to present as any clan with an appropriate family. This is somewhat hidden in the red sidebar on page 228. This little sidenote can be easily forgotten since it is placed right next to the introduction of the imperial families.
UNUSUAL SCHOOLS: [...] Kitsune who dwell in human society mimic the traits of the family they join, so they choose a clan and family normally.
As a result, you should understand the 1st question of character creation not as "What clan does your character belong to?" but as "What clan does your character claim to belong to?" and the 2nd question "What family does your character belong to?" should be read as "What family does your character claim to belong to?". The answers are handled the same as normal in character creation, despite the understanding that you aren't actually a human that grew up like your peers but a fox that plays the role of them.