It is not officially possible to convert a Kaspersky activation code (a 20‑digit alphanumeric code) into a .key file using legitimate means. Kaspersky moved away from .key files years ago in favor of activation codes and My Kaspersky account‑based licensing.
I understand you're asking about converting a Kaspersky activation code into a key file, likely because you've come across an online tool or guide claiming to do this. Here’s the short version of the story behind that search: Convert Kaspersky Activation Code To Key File