1. is it possible that only registered users can apply for jobs ?.

Since there are no accounts on jobberbase, this is not possible
out-of-the box. But it's possible to extend the functionality and add
some code that does just that.

2. can i integrate the user database with joomla database ?

As mentioned above, there are no user accounts in jobberbase. So you
could use the joomla db directly, with some code in jobberbase to
support it.

3. can i maintain separate account for employers? ( To post jobs )

Doable, but not out-of-the-box.

4. need to add few fields in jobs?

No problem, but you need to code it.

I need code for all the above