After a small search I found a great tool named ILMerge.
Main purpose of ILMerge is to combine more than one .Net assemblies into one assembly. The side benefit is that you can also sign the resulting assembly. It works while working with only one assembly too. This way you can sign an assembly without having the source code.
Sample command line:
ilmerge /t:library /keyfile:"c:\SomeKeyFile.snk" /out:"c:\signed\assembly.dll" "c:\unsigned\assembly.dll"