No, once the margin call starts, the execution of all new orders will be rejected, as there is no sufficient collateral to increase your exposure further. You will only be able to reduce. Be aware, however, that if you have multiple positions netted via FIFO (first in and first out) on an underlying, it can become very difficult if not impossible to reduce your exposure. In fact, some orders to close could imply an increase of the exposure, instead, and, therefore, the system would reject them.