NO!
Do not create a copy of the standard SAP transaction! Reasons:
- every note applied to your SAP system that corrects errors around ME5* or ME2* will totally bypass your custom made transaction.
- every upgrade will adjust all of your SAP system but bypass all the clones. If you don't buy a software or SAS that helps you pre-upgrade to identify and adjust all the clones in your system, you're practically doomed.
In your case, first look, if there's a BADI, because you would need to check on the authorization at the point where the user enters a cost-center in the PO or PR. Since this is dialogue, a BADI sounds like just the thing.
If that can't be done, try for an user-exit.
If that also can't be done, modify the SAP standard. This is the best way, believe me, since every upgrade will throw up your modification in SPAU/SPAM, so it will never go unnoticed or get "forgotten" and your adjustment will be obligatory.
As for Mehmet's hardcopy. Authority check for G/L accounts could only work in your case, if you bind the cost-centre to a G/L account. This can easily be done but you would have to clarify that with your FI people first. If they agree to make the accounts cost-centre relevant, this might be a good approach. Test it out!