If you wish to brick your phone - yes.
Otherwise - no.
EDIT: On a serious side - it is highly likely that you will brick your phone (or lose most of its functionality). Better not try that. Custom ROMs is fine, as they only change system partition, however stock firmwares include kernel, recovery, modem etc also.
Current devices: Samsung Galaxy Note 3 (N9005), Sony Ericsson Xperia Arc S
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