Creatus WordPress theme comes with extended Unyson framework extensions that you can take advantage of. You can extend these extensions at any time. If an extension is located in Creatus framework extensions folder, the Creatus child theme is able to override the extension files by copying them in to child theme corresponding folder;
├─creatus/ │ ├─inc/ │ └──thzframework/ │ └─extensions/ │ └─megamenu/ │ ├─views/ │ └──item-link.php │ └─creatus-child/ ├─inc/ └──thzframework/ └─extensions/ └─megamenu/ ├─views/ └──item-link.php
Once you copy the files over you can make desired changes to extension files and extend your creatus-child theme extensions.
Creating custom extension
Creatus theme is built on top of Unyson theme framework and you can take advantage of all it's extending features. To create new extension from scratch follow this useful Unyson Extension Cookbook instructions.