1) How do you enable SIP on Nortel CS1000 and Nortel Signalling Server? If required license, how much it cost?
You really need to consult a voice reseller... you need to be running at least 5.5 or later (highly suggested to be running 7.0 or later) and you need to purchase SIP trunk and line licensing.
2) Currently we are using Nortel IP Phone 1140e running firmware 0625C7J. Is it possible to convert 1140e to be SIP-enabled phone? Can it run both SIP and UNISTIM? If upgraded to SIP, is it possible to revert back?
Call Server release 5.5 or later can run both UNIStim and SIP IP trunks at the same time... again you need the licensing to support it.
3) Can Nortel IPSoftPhone 2050 run on iPhone, Windows Mobile, and/or Android devices? If it can't, is there 3rd party Nortel IPSoftphone 2050 client for the Mobile Devices?
In theory if you upgrade to Call Server 7.5 and deploy Avaya Session Manager you should be able to use any third-party SIP client.
Unfortunately the fact is that SIP is not really the great standard it was supposed to be. You have Nortel SIP, Avaya SIP, Cisco SIP, etc, all of which are not compatible with each other. Now enter Avaya Session Manager or Cisco Session Manager, these two offer abstraction layers to help merge the different SIP protocols.
Again you really need to consult a voice reseller, it's very complicated stuff and there are no manuals.
Hopefully I've answered some of your questions.
Good Luck!