Sure can..
Select your slideshow, then hit the
Export Selected Clips. You will be presented with a menu. Choose one of the format options. However keep in mind that you want to mix this with a video file, so instead of having a field order of Frame Based, it would be better to use the same field order of the video clips you're going to join this with. For example if your video clip has a field order of Lower Field First, then you would want your slideshow to be the same.
To export your slideshow, and be able to change the field order, you need to select
Customize (bottom option). Then once the Customize Dialog opens, choose a name, then hit the
Options button. In the Options Dialog, on the
General Tab, change the
Frame Type to match the video clip it is going to be joined with. Click OK, Click Save.