MacJournal can import any file that you want and store it alongside other entries that you create inside MacJournal. To import a file, you can drag the file from the Finder to a journal in the Sidebar to add it directly to a journal. Alternately, you can use the "Import" menu in the File menu.
When importing new entries with the "Entries" item, MacJournal will ask you which files you want to import. For text files, MacJournal can try to separate multiple entries in the same file. To try this, click the "Try to Discover Entries in file" checkbox at the bottom (if you don't see the checkbox, click on the "Options" button). For this to work, the file must be formatted in a certain way. Each entry must start with a line with "Date: " at the start and the date of the entry right after it. The next line can optionally include a line with "Topic: " to specify the topic for the entry. For a good example of what MacJournal is looking for, take some entries thatoyu have and export them to a single file and add the Date and Topic metadata.