Is it possible to embed a pre-existing DLL into a compiled C# executable (so that you only have one file to distribute)? If it is possible, how would one go about doing it?<
To expand on @Bobby's asnwer above. You can edit your .csproj to use IL-Repack to automatically package all files into a single assembly when you build.
Install-Package ILRepack.MSBuild.Task
Here is a simple sample that merges ExampleAssemblyToMerge.dll into your project output.