Is there a reply to all option in Kmail?

Yes, there is. The default reply button in the toolbar needs to be clicked and held for a second to show all the options (see the small downward pointing 'v' symbol?).

You can right click on the toolbar and add a separate reply to all button, if you like. You can also, of course, simply right click on a mail and choose reply all in the reply sub-menu.

or you just hit the "a" button on your keyboard... just as "r" is reply, "+" is next unread, "n" is next mail", "p" is previous mail. These shortcuts are very handy and very poorly documented.