You should be able to use a flat-bottom bit to drill the hole very carefully and not damage the tank shell, but threading it may be tough unless there is enough repad thickness such that you can get sufficient threads cut without bottoming out the tap. If you can't tap the hole, you could weld an NPT coupling to the outside of the repad. It would help to know the repad material and thickness and if it was PWHT.

Thank you IRFr. I will inform our assembly supervisor to try to install a half-coupling or a nipple in order to do the test after the drilling procedure.

Why bother tapping the hole unless you want to air test the repad?

The secondary use of the tell tale is to allow site operators during routine maintenance inspections open the plus to quickly tell if the internal shell to nozzle weld has cracked. Product or water ingress which could lead to crevice corrosion can then be defected - as well as any defected weld.