Yes, it's true. Conversate is not a word. Now please stop using it in conversation.
See, converse is the actual verb you're looking for. Conversation is the noun. You can see where the slang term conversate came from - it's just a bastardization of the noun form of the word.
That being said, while fully I agree you shouldn't be using conversate in daily conversation, that doesn't mean you can't use the slang term in your writing. There are times when that word is completely appropriate, if you consider the context of your work.
For example, it could be a couple of teenagers talking. It's a fairly common word for them today. Also, the word pops up in the slang of African Americans. Or, it could even be a college professor yelling at his students to stop using words that don't exist.
Just remember that when a writer uses slang, it says something. Slang always has a particular feel and time and even culture. When your character or narrator uses slang, you instantly date the story and set the setting, perhaps even influencing the tone. Word choice, or diction, is a potent thing.
Slang, in other words, is a powerful tool in the writer's craft. But like most tools, it can be quite harmful when used improperly.