In the link below you will find a mod that will allow players to take Shar-Teel as a potential party member in Siege of Dragonspear.
She is only acquirable during the games prologue. You can find her in the Flaming Fist's headquarters in one of the jail cells.
She has only limited dialog during the game (and it is all in modal form, so no overhead text). This will make her stand out compared to other NPC's. I hope to add more dialog for her in the future and maybe look into converting more of her banter to the overhead text format SoD uses.
So bear in mind that this mod is still very much a work in progress.
Also, if you are starting a new game in SoD and wish to dual class her I've scripted her so that she should allow a degree of flexibility with this. Once you dual class her (and return to the main game screen) you should find her gaining some experience. Whether or not she does (and by how much) is based on Player1's XP. If you do want to benefit from the XP (when it comes to dual classing) you'll need to do it while you are still in the main floor of the Flaming Fist headquarters.
(to be clear here you can't level her up from the XP boost she gets when she joins your party, and then dual class her, and expect the extra xp)