Variables to work in Templates

[Logikos]

I would like to add a new variables in a few diffrent templates,

Example
i would like to make a new template for the "Forum Home Page Templates" So would i just create a new template called forumhome_offtopicbit with my html code inside that, and add $offtopicbit anywhere in the forum homes section?

Or does this reguire adding a code to forumdisplay?

if so what code do i use?

and if i want to make it gobal do i add the same code to phpinclude?

thanks

-Nem