Hi SimonLenz,
As per my understanding, you want to use the .xaml file which is already created and use that workflow content in your new project.
The best way of doing this:
If you are creating a new project and you want to use only one or couple of .xaml files from previous one, you can just copy those files and put it in your new project folder. If you are doing modifications to the existing project, you can just open the previous project and do modifications accordingly instead of creating the whole new one again. Copying all the (activities) content from old .xaml file is little confusing and some times there are changes you miss some activities if you are not copying correctly.
If you copy the process (i.e Sequence\Flowchart) it wont copy the arguments, so you can do one thing.
Right-click on the sequence you need to re-use, and click export to workflow and later you can call this in your new process
Pardon my frankness and excuse me if I’m overlooking something, but why isn’t there a basic “Save as” functionality that allows you to quickly and easily save a copy of the entire project? Pretty much every other program in existence has this basic functionality. Why does this need to be so complicated a task?