Should I extend a drain vent that terminates in my attic?
Bad. Not only are you introducing stink and moisture to your attic, but toxic and potentially explosive gases as well. 


  Decomposing waste materials in public and private sewer and septic systems create sewer gases. Methane is the largest single constituent of sewer gas, which includes an assortment of toxic and non-toxic gases, such as hydrogen sulfide, carbon dioxide, ammonia, nitrogen oxides, and sulfur dioxide. Improperly disposed gasoline and mineral spirits may also contribute to sewer gases.
  
  https://www.nachi.org/sewer-gases-home.htm?loadbetadesign=0


Fix it now.