one way of quickly getting another project's tempo map into the own project is this:
- in the original source project, enable snapping, create a new track and insert an empty MIDI item of the length of the entie song (maybe even a few bars longer)
- insert one MIDI note into this MIDI item at its very beginning and another MIDI note at the very last beat
- click on the MIDI item to highlight/select it
- go into the file menu and select "export project MIDI"
- make sure to check "entire project", "selected MIDI items", "merge to single MIDI track" and "Embed project tempo/time signature changes"
- define a file name and a location to export and click OK
- import this MIDI file into your project; there should come up a dialog, check "Import MIDI tempo map to project tempo at 1.1.00" and click OK
This procedure should preferably be carried out before starting to record or inporting any audio items.
EDIT: Unfortunately, this procedure seems to not always work perfectly if tempo map is fairly complex and includes features like smooth tempo transitions and unconventional time signatures.
I first thought about simply creating an automation item of the entire tempo envelope. This would be the most convenient way. However, master track doesn't allow to create an envelope from it's tempo envelope. I just filed a feature request for this.
.