XDA member ktoonsez has developed a Windows application named Flash-aholicsTool that will take a ROM for the international version of the Galaxy S II (I9100) and port it to work correctly on the AT&T Galaxy S II (SGH-I777). The tool will only work for Gingerbread Roms (Sorry no ICS ROMS). This process is called “Hellraising” a ROM. The tool was based off XDA member Entropy512’s procedure for converting stock Samsung I9100 ROMS to work on the I777 which entailed flashing the I9100 rom then flashing a separate zip file immediately after. ktoonsez developed this process further to not only include stock Samsung ROMS but AOSP ROMS as well and made an intuitive UI that streamlines the process. A day after the tool was released “Hellraised” ROMS filled the i777 xda forums.
The tool is simple to use: You select the ROM you want to convert, select a compatible kernel and modem file and then click the “Hellraise Rom” checkbox, click “Build Rom” and then take the output zip and flash it it recovery. There are also other options you can add in like build.prop tweaks and init.d scripts I have used the tool to get the latest version of MIUI (2.2.3) when only 2.1.13 was avalible for the i777 and it worked great.
Head over to the thread on XDA for further information: [MOD] Flash-aholicsTool
Entropy512’s original Hellraiser thread [MOD] I9100 ROM Hellraiser for I777