I think it is
better to update using jdb file 

For updating
using jdb file you can use below doc

How to update definitions for Symantec
Endpoint Protection Manager using a JDB file

http://service1.symantec.com/SUPPORT/ent-security.nsf/docid/2009072204590148

you can
download rapid release updations from below link

http://www.symantec.com/business/security_response/definitions/download/detail.jsp?gid=rr

I m not sure whether this work or not..
Give a try...

According to me rapid release updates are available in ftp site...

ftp://ftp.symantec.com/public/english_us_canada/an...

so you can write a ftp script to log in to  ftp.symantec.com (anonymous user name password  should work)
then mget  *.jdb file...

you have to make sure you will create a ftp script and batch file inside "Program Files\Symantec\Symantec Endpoint Protection Manager\data\inbox\content\incoming" folder. Schedule the batch file to run any time you want...

------------------------------------------------------------
create a file rapid_download.scr and put this...(Use notepad to type and save as rapid_download.scr)

anonymous
anonymous
cd "public/english_us_canada/antivirus_definitions/norton_antivirus/rapidrelease/"
mget *.jdb
quit

create a file Rapid_download.bat and put this.

@ECHO OFF
ftp -s:ftp_get.scr ftp.symantec.com
exit
-------------------------------------------------------------
I have not tested.. because i don't have ftp access in out setup... just an idea... you can test it and tell me... 

Hi found this... i think it should work for you after little modification....

Using a modified Cegetter utility for downloading Rapid Release virus definitions for Symantec AntiVirus Corporate Edition
http://service1.symantec.com/SUPPORT/ent-security....

I m not able to download the script to modify for you.. .
But modification in the ftp script(to get jdb instead of xdb) and changing the path to which jdb will be copied should work fine....

I've posted such a file here that FTPs to Symantec, checks for a later file than it last downloaded, and gets one if it exists.
Check my uploads area................

 https://www-secure.symantec.com/connect/downloads/file-download-jdb-files-sem

After updatiing from from rapid release willl i have any problem with using live update after that.

 No you won't have any issues after that...RapidRelease Definitions are mainly for Emergency situation or in a Closed NEtwork ( without internet). So once new definitions will arrive the Liveupdate will download it.