You don't need to follow the texture naming religiously. a normal map with the extension _CO will work perfectly fine too. All the file extension does is tell Pal2Pac how to compress your texture to save file size. (ie nuke entire channels to be one color). equally you can use _NO for super low file size normals, _NOHQ for regular and _NOVHQ for basically 1:1 reproduction. if push comes to shove you can also add this to your texConvert rules and make it uncompressed:

class Uncompressed
{
	name     = "*_uc.*";
	format   = "ARGB8888";
	dynRange = 0;
};