The easiest method would be to rewrite the original method like so:
void IsNotNull<T>(T obj){
IsNotNull(obj, "obj is null");
}
Then click on the method signature, and choose Refactor->Inline Method (Ctrl+R, Ctrl+I). If you need to keep the original method signature around, you can, or you can check the box for "Remove inlined method declaration."
EDIT: Looks like hmemcpy's solution works in 7.1, so I'd suggest an upgrade. If that's not possible, however, try the following regular expression find-and-replace in Visual Studio.
Find: IsNotNull\(([^\),]+)\);
Replace: IsNotNull($1, "$1 is null");
Make sure "Use Regular Expressions" is checked, and "Look in:" should be "Entire Solution".