Great question. "Honorary" and "honourable" (or "honorable" in U.S. English) have very different meanings.
1. An honorary degree is a degree awarded without being earned in the conventional sense. For example, a famous person giving a speech at a university might be given an honorary doctoral degree (i.e., Ph.d.) without ever having taken a class or passed an exam. Synonyms for "honorary" would be "symbolic", "ceremonial", and "unofficial"--in other words, it's not a real degree. Think of it like giving a dignitary the "key to the city"--it's not a real key that actually opens anything.
2. "Honorable", by contrast, means "worthy of honor or respect." "The Honorable" is a title often used when referring to judges or other high officials, as in "The Honorable Justice Smith." This word is also commonly used an an adjective, as in "The honorable thing to do would be...", or "An honorable person would..."
So no, you can't use the words interchangeably.
I hope this helps. Good luck!