Unfortunately like many Panasonic models it's not easy to hack the region coding.
To do so requires a special engineers' remote control, and Panasonic only supply them to officially approved service centres.
Fortunately, there are some companies which will allow you to hire the correct remote control, but beware: some have a single use only, so if you get it wrong you have to pay again. Others.,
like this company, send an unlimited use remote set up for the exact model you wish to use it on. The fee charged costs more than the cheapest and easily hackable generic DVD players, and the deposit on the remote against it's safe return is high and requires you to have a credit card.
Please note that the link may only be of use if you're in the UK, if elsewhere you'll need to search for your own local supplier. Just Google "Panasonic SC-HT900 region hack".