Hello @Oscar Garcia,
Could you please attach sample files (Input & Expected Output Format) to the question, so that the community members can try to repro the situation and help better?
I was able to reproduce your issue with a sample Source Data!
Then I have used the Derived Column Transformation+ Expression Builder to convert the my Source String Data to the required format!
If your Source DateTime is Read as a string then the below expression works, I hope this helps with your initial query!
toTimestamp(LastProcessedChangeDateTIme, 'yyyy-MM-dd\'T\'HH:mm:ss\'Z\'')
Please do let us know if you need further help in this matter.