Update Lead object field using Velocity Script

This is not possible with the current features.

What you can do is use a webhook to send the lead info to some code, do the processing/transformation and then map the response to lead fields. You would need the ability to host a script to send the lead info to, but other than that it's usually feasible for most users.

If you don't have the staff to host the script, there's a tool called FlowBoost that should be able to do this

Create a Webhook - Marketo Docs - Product Documentation