a) It is possible to share only user but with difference joomla extension which may not present in the master site?

b) Could i install a slave site(normal joomla installation) and later using the multi site ext to setup sharing user?

a) You can share the user with a website site that is not the master.
See our tutorial video 16 that show how to share the users between slave1 and slave1A

b) NO.
a fresh Slave site that consist in executing a joomla installation result by the creation of indepedent users..
If you want to share the users, this must be done with a replication of an existing website.
You have to define in the JMS Templates that you want to share the users with the "template site" that you have selected to build the replication rule.

I have watch the video....but i still confuse whether yours multi site are suitable for us to use on our exiting site.

I looking for it to share user on my exiting site. Which i would like to create a sub-directory or sub-domain that share our user from our exiting site but with difference joomla ext.

But you say that I cannot share on the master that mean i can't install on exiting site...is there a solution for this?

You can share the master - this is just not recommended
In the tutorial video 16, rather than to create a speciific slave site where all the user will be stored, you can use the master website