Q. Is it possible to create a 2012 R2 cluster in a 2003 domain mode AD domain?

A. If you have 2003 DCs then get rid of them! It's an unsupported operating system and you definitely don't want such a critical component of your organization unsupported and susceptible to attack. Replace them with 2012 and above DCs and if there is some add-on that requires 2003 DCs then replace that software with a version that does not require a 12 year old operating system.

That being said if you have to keep 2003 DCs you can create 2012 R2 clusters but you will need to install a hotfix that is one of the recommended updates for clusters.