Hmm, I'm afraid it's not always as simple as that... here are some pointers to think about:
1. Copying the script - be careful when combining the two data models. You really need to ensure that the two sets of entities play well together and don't create any problems with multiplicity, loops etc... This is perhaps one of the most important reasons for not being able to offer such as merge "out of the box".
2. Copying all the variables - yes, true. In addition to variables, additional entities may need to be checked:
- Actions and Triggers
- Alternate States
- Custom Number formatting settings
3. Copying the sheet objects:
This is perhaps the most trivial part - Ctrl-A, Ctrl-C to copy, Ctrl-V to paste in the new document. Keep in mind that you may have some conditionally hidden objects. If you use conditional show/hide, all the conditions need to be turned on one by one and the corresponding objects copied individually. Alternatively, you can press Ctrl-Shift-S to toggle "show all" setting. This way, you can copy all the objects at once.