Not just a boss but "a mob".
To test this, use any of those 'goddess' maps (102, 104) so you don't mess with maps that players are using. For example mapa 102 (goddess 1-15).
Edit the svmap in shaiya studio and put 1005 on the "Faction" field on one of the portals.
Then in Obelisk.ini add a new correlative entry:
[bothmob_0XX, (102, 1, 9000, (102,1005))] = (918, 1.5, 918),(2009)
That should create a Thief Monkey, wich you should kill to open the portal you made '1005' faction on that map.
(I dont know how much time the portal will be open)
Be sure to backup your files before testing this, in case of errors.
And you can read more about portal conditioning on this post: