Mustafa B. - I've trained such a tool myself on OpenAI - using Projects and uploading my existing content, brand tone of voice as context. This is pretty simple to do, happy to show you and give you some prompts to use. Think the best way to start is just to start.
But if you need to build something that can be used by multiple people securely on your team - you'll need do get something a bit more custom -- I run a company that's current exploring this use case -- where we can generate campaign ideas but also make sure those are grounded based on real data and understanding how leads are behaving down funnel during the sales cycle in your company. I'd be happy to get your perspective and ideas on this concept if you like.