As in this example:
This can be accomplished by creating any third note in the middle of the set and setting it (and it's stem) invisible and it's velocity to 0 (so that it doesn't sound), then creating a set of eighth rests in voice two and repositioning them. Don't forget to set unnecessary rests in voice two as invisible as well, i.e.:
As in: http://i.imgur.com/3p7STY3.png
Right click the measure, click "Measure Properties" - leave "Nominal Duration" alone but set "Actual" to the true length of the cadenza. Click the first note of the cadenza, hold shift, then click the last note to select all notes in the cadenza. Right click one of the notes, click "note properties" and then check the "small" box.
For cadenzas that continue onto another system (i.e. http://i.imgur.com/2Eh7EPs.png), simply set the measure line at the end to invisible.