When installing a package through the ports collection,
is it essential that the tarball of the exact version
of the package should be present under the distfiles
dir?

My FreeBSD box can't connect to the internet, since
my modem is internal. Hence I'll have to download
the tarballs using Linux, then move the same onto
distfiles dir, to install using ports.

Kindly clarify.

comp.unix.bsd.freebsd is not widely propagated, so you won't
necessarily get much help here.
See http://www.freebsd.org/support.html for better ideas...
<followups redirected accordingly, if I remembered the syntax correctly>

Not necessarily, but (a) you'll have to override the checksum step
(with NO_CHECKSUM), (b) you won't have the checksum protection against
corruption (intentional or otherwise), and (c) the port may not work.

You should be able to get the exact files you need.  See the
"fetch-list" target under "man ports".