Override single strings of PO files

Is there any way to override specific strings when using PO files? With arrays it’s possible to merge them, so the application can have just the strings which require a replacement.

I found this solution but will override the whole file, so if strings are updated/added to the upstream file they’re not added locally.

Any way to do something similar with POs?

why not update the file?

Because the file is an extension in vendor, and I’d like to use a custom translation despite what the upstream module has.

Which extension. If you keep posting half cooked information, you are not going to attract much help!