Hi I am a beginner in SMath. Is it possible to install plugins to SMath in an offline fashion in a computer not connected to internet.

Yes, you can download the portable distribution with pre-installed plug-ins. Follow the link in my signature.

Also, you can download the plugins from here, store them with the installer, and after you install the program you can drag and drop the zip packages in the program canvas. The downside is that you have to be careful with the compatibilities, if the program version isn't the latest (f.e. if you download the stable version and nightly builds are available).