Hi Everyone - definately sounds like an important capability, hoping Hubspot will roll it out asap.
Meanwhile, I think a simple workaround can be as follows;
1. Add an extra field for each of thos you don’t want overwritten, for example Lead Source, make Lead Source Temp.
2. Map the import to Lead Source Temp
3. Create a workflow - if field Lead Source is blank, copy Lead Source Temp to Lead Source.
You can do a similar thing for fields to append, make Notes Temp, then make a workflow - if Notes is not blank, append Notes Temp to Notes, if blank - copy Notes Temp to Notes.
How does that sound for now?