Adding additional differential backup on your existing backup policy may impact your performance(schedule it at non-peak hours).. and not to mention,you will have to preserve it to achieve the recovery..
Thanks & Regards
MCP,MCDBA(SQL 2000),MCTS(SQL 2005,2008),MCITP(SQL 2005,2008)
"Most Good Experience Comes From Judgement,
Most Judgement Comes From Bad Experience"