@Sojkich
I and @SudeepGhatakNZ can understand what you want to do but in the admin setup
https://learn.microsoft.com/en-us/connectors/office365users/
there is a place for Birthdays, see below:

If you have the lsit of users, with their birthday then ask your admin to add these to the user account. Then for all new users this could be added at that time.
Getting back to what you want to do, I would suggest things like birthdays need to be stored in one place and that is in the user account information above. This makes it secure and not requiring a seperate excel file to store it that could get deleted etc. And as you said your company is a bit strict on things so I would assum personal information like this would need to be stored in the user account. Ad if this is the case if could drive a lot of other activites in the company. As @SudeepGhatakNZ was trying to suggest if this method is used then we would use the user information to drive the flow you want.
If you still want to use the excel file then please advise.
Regards,
Andrew