I want to add an inner fore stay for a storm jib.

Go and speak to you local friendly rig shop. Considering the word ‘storm’ one hopefully understands what that means. Get advice from a professional or the mast manufacturer.
You may not know something they do and any ‘tinkering’ with the spar may result in damage or failure – resulting in a potential life threatening situation and a group of lifeboat volunteers having to put their lives on the line just because it seemed to be OK to drill holes, install brackets or create lashings that either failed or caused a failure.
Advice from the spar maker or a good rigger, even if it means opening your wallet to buy a few beers will be the best course of action. If they say no then there is a reason. If its a yes, then get a deal done and get the rig shop to do the work. It will be worth the financial outlay – added piece of mind and less litigious issues if , god forbid, something goes wrong.
Remember the word used ‘storm’ – it means more than just in a T cup.
Your insurance company, if you have one, must be notified…they do not take kindly to DIY’ers mucking about with masts…
At the end of the day it’s your call and you did ask for advice…safe winds for your project…