There are no names in this website that have begin, end or have 'stag' or 'staj' in them so that their nickname may be stage. Your sister could pick a name of a famous dramatist - try searching for drama under description and using male names only:
http://www.behindthename.com/php/search.php?terms=drama&nmd=d&gender=m&operator=or
Or perhaps she could try to take one of the names of the Greek muses, one that is related to theater or drama and try to make it masculine. Here are a list of the muses listed on this site.
http://www.behindthename.com/php/search.php?terms=muse&nmd=d&gender=both&operator=or
And here is a complete list of the muses on Wikipedia:
http://en.wikipedia.org/wiki/Muse
Other members of this forum may be better suited to helping you take Greek feminine names and making them masculine in line with ancient Greek styles of naming.