For example, add x.dll to the project and set its Build Action to Embedded Resource.
To extract:
string AppPath=Assembly.GetExecutingAssembly().Location;
Assembly ThisAssembly=Assembly.LoadFrom(AppPath);
System.IO.Stream fs=ThisAssembly.GetManifestResourceStream("yourproectname.x.dll");
int ssize=(int)fs.Length;
byte [] buf=new byte[ssize];
fs.Read(buf,0,ssize);
fs.Close();