I was wondering , if anyone know whether I can take just a GRE course for a sem to maintain F1 status .

 Yes you can if they issue an I-20!!! l
You must be talking about KAPLAN right , where they teach nursing courses , GRE , GMAT and so on. 
The one based on Dallas used to issue an I-20, but they don't do anymore for GRE. But the one in Houston does, so as long as u have an I-20, you are good to go dude!!! 

 Its not extending an I-20. Its like they will issue an new I-20 from KAPLAN. You need to go by person, so far I know. And there are certain rules to maintain the F1 with them. Like, you need be @ KAPLAN (I mean teaching center) by person certain hours in a week.... to maintain your F1. 
I would suggest you go by person to the center, and they should be able to helpyou out. Seems like you are trying not to attend any classes right? 
Its going to be lil hard.... for that reason do the following:
English learning center ma jon garne.... ani teha ko english sajilo ni huncha... tini haru le esai I-20 issue ni gardincha... alst ma exam dina jane... ani pheri transfer garne kata garne ho.... natra bhane KAPLAN ma every hours count huncha... login gare dekhi logout gare samma.. its lil strick.. so far I know....