Depends on how the atmospherics were made really. If it's a synth sound, then have that synth in it's own pattern. And make sure you are set to play the pattern instead of the song (up next to the time display there are two check boxes, one for pattern, one for song). Then go to file / export, and export it to a wav file. Then drag and drop that wav into slicex, chop the pieces you want and repeat the process of exporting to wav. Then re import the new sliced bit and viola.
If it's a sample, then drag it straight into slicex and chop, then export, and re-import.
Alternatively, you can drag and drop the wavs onto the audio part of the track ( where all the automations happen) and use the slice tool on the actual wav, then delete / move the bits you dont want.
im of the old fruity slicer paradigm. load up whatever you want to cut in the slicer, cut it up, drag that slice out to a normal sampler and have your way with it. problem with this method is that the sample becomes a file in a temporary directory not located where the rest of your stuff is, so remember to either save the sample as something in your own directory, or export project files when you save.