ms project training course - MS Project 2000

I have a large existing MS Project 2000 document with approx 1200 lines, I need to add another part to the project. This extra piece has been set up in MS project by someone else. When I try to copy and paste this in, or if I try to import it as a project, it changes the dates on the overall project.

Is there any way I can avoid this? I have to add and track plans in the large programme that come from various sources.

All the calendars and versions are the same.