in this week I have met at least two people combining ClearML with other tools (one with Kedro and the other with luigi)

I would love to hear how/what is the use case 🙂

If I run the pipeline twice, changing only parameters or code of taskB, ...

I'll start at the end, yes you can clone a pipeline in the UI (or from code) and instruct it to reuse previous runs.
Let's take our A+B example, Let's say I have a pipeline P, and it executed A and then B (which relies on A's outputs).
Now I have B' (Same B with newer code, for example), I can clone the orinial Pipeline execution P, and set it to "continue_pipeline: True" so it will reuse previously executed Tasks (i.e. plug them) and only run the New Tasks.
Does that make sense?

do you think it will be easy to modify programmatically the behaviour, by extending the pipeline class for example?

Sure, that is is the intention, and if this is something you think will be useful I'm all for PR-ing such features.
I'm assuming what we are saying is, the "add_step" is checking weather the step was already executed (not sure how exactly, of what i the logic), then if it is already "executed", plugin the Task.id of the executed Task. Is this what you had in mind?