When making High-Temp mortar, can I use Portland Cement that already has Lime mixed in?

No.

While some brands of Portland Cement contain some lime, it does not have the proper mix ratio. You need to make sure that your Portland Cement is lime free and then add the exact amount of Lime as specified in our instructions. Do not buy Portland Cement w/ Lime already added.

Or Diet Coke with Lime - it’s gross!