According to https://community.safe.com/s/article/working-with-geodatabase-field-aliases , when reading aliases from a GDB, an additional attribute with fieldname_alias is created, containing the alias. When writing to a GDB, this field is used to write the corresponding field's alias.

So I figured formatting an Excel table like below would work, but the fields are written to the GDB normally, and there don't seem to be any options to flag the fields as alias fields.