Convert fill to dicome

Hi everyone, I want to convert a set of files to di come
Is there a way to select a set of slices and get an output?
And I have to do another series of image that I should rigid registration between ct and mri images and I have to convert the files to nill and put all the slices in a compressed file.
Could me give guidance for convert alarge of image
Because I couldn’t select and convert a lot of slice and a only can select one slice and then convert to another
And any other guidance for rigid registration between Mr and CT with slicer elastic
Thank for all
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