how to merge two solid parts in catia
If you have two bodies then you can use the boolean feature. Right click on the body you want to be the main body and choose "define in work object"... then right click on the body you want to merge and choose "body obect" (at the bottom) then choose "add" if you need further help i can add screen shots if you would like.
Also you cannot add the "partbody" to another body first you would need to highlight everything within the part body and right click, then choose "insert in new body"
I hope this helps