But once I have imported the photos into iPhoto, I can open them into Mac TravelRecorder anymore.
Just Geotag your photo's before
importing them into iPhoto.
The 'finder' of MRT stops at the library and doesn't resolve the library or its files containing.
There's a generic trick for that. Browse to your iPhoto library using the Finder. Control click the library and select "show content" (or something like that). Now locate the directory of your photo's.
Next activate the directory browser of Mac Travel Recorder. Now drag the directory containing your photo's from the finder to the directory browser of Mac Travel Recorder. That's all.
But, the best way is still to geotag your photo's before you import them into iPhoto.