apply an external php code within wordpress

Hello !
i have a complete and external php pages(admin side and guest side), and i want to merge them with wordpress. i want to include the admin side within the admin control panel. and view the guest side as a pages or anything inside the wordpress .. is there a plugin or any thing i can do to apply that ?
i found out that the best way to write a plugin .. is there another way ? or should i start converting these pages into plugin ?

hope u can advice me .. and thanx anway =)