So you actually just want to let the user import some data (from excel) and put this data into a tag?
For this case you probably want to check this,
https://docs.inductiveautomation.com/display/DOC81/system.dataset.fromCSV
or
The gateway acces depends on the user rights it runs on,
Shared folders require some extra configuration to have access Mapping a Network Drive - Ignition User Manual 8.1 - Ignition Documentation