Apart from the whole segments, CafeTran can also add selected fragments, that is, words or phrases, to the translation memory (TM). First, make sure that your TM has the "Fragments memory" option activated in its context menu after the right-click at the given translation memory pane under its tab. Then just do the following:

1. Select a source language fragment in the source segment editor.
2. Select its translation in the target segment editor.
3. Click the "Add fragment to memory" button in the target segment editor's toolbar.
4. In the "New Fragment" panel, click the OK button to accept the selected fragment and add it to the translation memory.