Can i add "external id" to be in the forms? the only way i can update the value is by file or api, can i do it in the UI?

no, you cannot update external id via the UI. You can only update it via things like CSV, script, web services, etc. Although you need to think about whether you have integrations setting/using these external ids before you go changing them

Script is a better way You can create a field on your form then the external id will use this value to generate itself.

Thank you both