Speech recognition Plugin

Hi
i want to develop a plugin that can be used for speech recognition, for this i am using System.Speech.Recognition namespace,

this namespace is used in the visual studio by adding a reference from .net3.0 framework.

My question is how can i use this namespace or dll like i use it in visual studio C# environment, unity C# does not let the add reference option, can i place the dll in Plugins folder and start writing like i write in VC# e.g.,

using System.Speech.Recognition; // on top most part of the c sharp script

and then use this namespace's types and objects like i use in VC#.

my second question is if i write my plugin in VC# and build it as "Class library", can i use this dll in my unity C# script?
as per my knowledge we cannot use exter "C" in VC#

If there are other options as well, i would really like to listen.

Please help

Thanks