No can do at the moment. It has been mentioned in the suggestions box (http://www.opto22.com/community/showthread.php?t=101).
All variables used in opto script must be declared under the Variables dialog.
There is no stopping you from creating them in opto script, copying them to a temp txt file and adding them using the add variable dialog one by one by copy/paste. Saves you re opening the opto s riot block each time.