Yes, you can freely create your own words. Just as I can make up a word such as BAHGUTA (which means nothing as far as I know). But the question is why would you do so?For example 人問, which is not found in a Japanese dictionary, and I create this myself. How many Japanese words/vocabularies are there in a typical Japanese dictionary? The words/vocabularies were borrowed from Chinese, right?
Unless you're a marketer. Even today the word Fahrvergnugen comes unbidden to my mind even though it's a made up German word used in English campaigns.When you understand this basic fact, it's obvious why you would never want to "create your own word" from kanji, just like you wouldn't want to "create your own word" from Latin script.