Hi Badr,
You can achieve your goal either by a Javascript activity before the Extract file activity (or in the Javascript tab of the Extract activity, but less "visible"), in that case, add the node in the temp:schema with XML functions of AC API, or, for simple use cases, what I do usually is doing the conversion directly in the Format Data of the extract activity:
(cryptString or SHA256 or whatever can be done here easily and efficiently considering performance with high volumes):

So you have to add your fields (button New) and unselect the checkbox of the fields coming from the 1st page (query) of Extract activity:
And by clicking in Detail icon of the new added fields, place your Javascript code conversion.
Use the toolbar button list box to generate the source.target__XXX or type it directly.
Regards
J-Serge