Hi Ismael,
This does not seem to be a feature in Business ByDesign. Nevertheless, to what document are you trying to attach the signature? And have you checked your 'Signature' configuration in Fine-Tuning whether there are any hints whether this is possible?
Could you also please check IdeaScale https://ideasforbydesign.ideascale.com/a/ideas/search?query=electronic+signature as there seem to be several existing ideas that are relevant here.
Kind regards,
Douglas Kisinger Thompson
Support by SAP Cloud KM