You don't have to edit manually - that's why you have regexp. Just don't drag and drop - causes repetitive strain injury :-))) Seriously, you may have to let VS create that file at the beginning, but after that - strict text editor policy. Saves a lot of grief down the line.
As the commend bellow said - manual editing for hundreds of files is impractical - but writing a PowerShell script or a C# snippet to construct it isn't. The more files you have the stronger reason to script this. It will save you time even the first time around and latter you might even start using it as a part of the build.
If you have a good editor and no time for a script immediately you can also just do a recursive dir of wherever your files are and literally construct good part of Resx with a few regex-es.