There is a way to load the fields definition from external file?
No
A more detailed documentation could help
Source: C:\lazarus\fpc\3.2.2\source\packages\fcl-db\src\export
File: fpfixedexport.pp
procedure TCustomFixedLengthExporter.BuildDefaultFieldMap(AMap: TExportFields);
If you want "custom" Field-Definitions, you have to override (either the Procedure above itself, or set FieldTypes et al after Creation, if that is even possible)