If you are getting close to a margin call then you have done some poor money management on the trade. You have done one or more of the following:
1.   You have over traded your account.
2.   You were trading without a stop loss.
3.   You have not set any limits as to how much you will let your account draw down before you close a trade
The answer to the question is a resounding NO, never add money to cover a margin call. In most cases you should just close the trades and take your losses. Better yet, you should have seen the writing on the wall and closed the trades long before it came close to a margin call.
You will probable notice that the first loss is always the smallest. That means that if you set stop losses or determine the amount you will let your account draw down then that is going to be a smaller loss than just hoping it will come back in your favor and letting it run.
So I guess what I am saying is to never get close to a margin call and if you do, do not add money to cover a margin call. That is throwing good money after bad.