How to convert an assembly into one solid model (single body) IN cATIA?

Follow these steps

Answered with a tutorial: https://grabcad.com/tutorials/how-to-convert-an-assembly-into-one-solid-model-single-body-in-catia

Open your assembly and select it in the tree.
Then, in the menu, select:
Tools -> Generate CATPart from Product...

Then you can choose if you want to merge all the bodies in to one body or have a multi body part.

Save it as a STP file

Open your assembly and just double click one of your assembled files and then save it you will have one part product.