You can use MethodDefinition.Body.GetILProcessor()
to receive an ILProcessor
object. That allows you to further modify the method body and inject whatever IL code you want to add.
You might get some basic ideas from Obfuscar.HideStrings
where new classes and methods are created from scratch,
https://github.com/lextm/obfuscar/blob/master/Obfuscar/Obfuscator.cs
There are other open source projects based on Cecil, too.