Can I combine TextEdit files w/o copying and pasting?
Credit:https://discussions.apple.com/thread/2242153?tstart=0

You can achieve this by opening terminal and typing cd [path of the folder where your journals are located] you should replace [ ] with the the folder where your diary entries are written , just drag and drop the folder into the terminal window.(Remember to hit space after typing cd) Then you can merge all the text edit files into one by typing 
cat file1.txt file2.txt file3.txt &gt; file4 The file1.txt, file 2.txt and so on will be your diary entries so if you have more just type the name of the file followed by a space and the &gt; symbol will merge all the documents into one file4.txt you can change the name as per your concern.