I would like to have this and several special characters in my team, and I would like to know the basics to be able to do it.
The golden rule to starting out on FFH is to open up the Patcher and look around at everything to get an idea of what you can do. If you look in the ENTD for each individual battle in the game, you'll see that units can be flagged to Join After Event. Now, that won't be a perfect resolution - their Formation sprites won't be correct, and there'll be no story explanation for why these characters just suddenly joined.
The second golden rule is this:
Thank you very much, I am new to this but I come with a lot of inspiration and I hope to contribute something in the future.
Best of luck and welcome to the community! The mods here are very helpful so long as you keep good manners, follow rules, and provide as much info as you can/attempt solving it yourself first. It's an arduous hobby but fun despite the steep learning curve.