...are possible with capella 7.
With older program version you must do a little magic:
If your staff contains two reasonably homophone voices, then you can add the normal text to the one voice and the italic text to the second voice. All you need to do then, is to adjust the spacing for each voice separately in menu format | voices (lyric verses) | lyric verses.
If you have only one voice on your staff: Add a second voice via copy and pasting from the first voice, while the original voice maintains its status as "main voice". Next, you format the second voice to be invisible and mute.
Then you attach text to the second voice and format it in whatever font style you wish.