You have to modify conversation.txt
Look for the dialogue that the heroes say when they join you. You have to copy that whole block of instructions (maybe 5 lines) and then paste it into the beginning of the conversation.txt file. Then change the numbers at the start of the lines. This controls the speakers ID. You might have to read up in the Mod Info: Conversation.txt to learn how these IDs work. Youll also need to increase the number at the very top of the conversation.txt file (the size) by the number of lines you added.
There is also a variable with each hero to track his status (in party, never recruited, recruited but left somewhere). You dont have to change this as you probably wouldnt know where the free variables are. This just means that you may have various problems when you try to leave that hero on the worldmap. So just dont leave him.