Welcome!
you will no doubt get many responses asking:
- why do you want warning limits? what action will you take if you violate a warning limit
- what kind of process are you dealing with?
- How stable and capable is your process?
- How much do you understand about the inputs? can you reliably use them to alter the output in a predictable manner?
Answers to these questions will help us give you the best answers that will actually be useful for your situation.
In general, we wouldn't use alert or warning limits on specifications UNLESS the process is predictable (the relationship of the inputs to the outputs is known and predictable) AND we have the ability to adjust the inputs to reliably achieve the desired output. If we don't have these two conditions we are most likely going to make things worse - this is known as tampering.
But to directly answer your stated question: the tool that fits this is PRECONTROL. Like any tool there is an appropriate time to use it and inappropariate times...I have attached a briefing on Precontrol that I use within my organization.