Say, you need to be logged into the site before accessing a vulnerable page, in other words use a session.
Generally
when you are logged, a cookie is affected to your browsing session and
it allows you to access other pages of the site.
In order to make
jSQL inject a browsing session, you should first create the cookie
session into your normal browser like Firefox and then retreive the
cookie ID affected to that session with tools like Firebug (e.g
PHPSESSID=eb9d471c8397f17558bb374d7720f19d). So, you can't avoid a real
logging phase in a way.
Then in jSQL, you define the url and
parameter for injection as usual, but also define the cookie you got
previously into the jSQL cookie parameter (expand the parameters with
blue arrow in the top right of the main window).
The injection process should therefore use your Firefox cookie and allows you to test other pages on the site.