That's a General question for the community who joins signature like me ,
Which makes it a bit off-topic here so people assumed you were asking about our signature campaign.
if our signature can be modified and can put referral link from Freebitco.in .
That depends on the rules the manager of the signature campaign you are in has set. If you are not in a campaign you are free toput your freebitco.in referral link in it.