Hi
With a single line text, leave a gap sufficient for the italic word/s. Set the position and justification and add the text frame (image) to the 1st overlay track (Video Track 2). Pull out the track so it has the desired duration.
Create you italic text image using the same font size/position etc. and place it onto the next overlay track (Video Track 3). Drag it out so it has the same duration as the normal text image on Video Track 2.
Preview the line up.
Adjust the gap in the normal text if necessary.
Apply the POSITION effect to the italic text image and in the effects preview window drag it about so it fits into the gap you left in the normal text. Note: Italic text requires a slightly larger space for the equivalent normal text, and some adjust may be required depending on the font chosen.
If you use several lines of normal text and want to insert several italic words, leave gaps as described above for each word and create a separate text image for each italic word. Add these to separate overlay tracks and adjust the position of each using the POSITION effect.. It's easier to do this than try to creat a single italic line with all the words spaced out and hope to make that fit.
e.g. http://i.pictr.com/dgd1v18lyt.bmp
Nat