What I would do is load the MIDI file containing your source tempo map into Sonar as a new project. Then open up the tempo map to make sure it's actually in there.
Then Select All and then hit CTRL-C (Copy Special) and make sure that only "tempo changes" is ticked. Hit OK. Now the clipboard should now contain the tempo map
Then open up the project that you wish to merge this tempo map with. Move the NOW to the start of the project. Hit CTRL-V (Paste Special). Make sure that the Tempo Changes is the only event type ticked. (All other event types should be greyed out as the clipboard should only contain tempo change events)
Note, you may need to open up the tempo view before doing the Paste to make sure there are no existing tempo changes that would confuse the project.
Hit OK
Open up the tempo view and your tempo map should be present there as pasted
Hope that helps