We are trying to change the values of some specific fields to the value of Custom Objects fields,
The value: ROAD SUITE to appear in the "first name" (or any other field) field value of the same user,
Is it possible?
We saw some posts about using Velocity scripts in some similar cases, is it the solution here as well?
That's not possible.
As you said, Velocity has access to Custom Object fields, but can only output them to emails. But not to "Change Data Value" flow steps.
As Michael mentioned, you can only read them and display them in an email but you cannot change them using velocity script or using change data value steps.