I never press aluminum headsets by the outer lip of the cup. It's too easy to distort or crack the cup this way.
If you don't have the adapters, you can safely press through the bearing, since the pressing force will be less than normal stresses in use. Don't over press at the finish, and do the lower cup second so if you're not sure you can leave it about 1/2mm short of fully seated. Riding will finish seating the cup and the only drawback will be having to re-adjust the headset once or twice as it settles.
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