Hi Sandra,
In principle this can be done and you can have your custom status based on who is approving and it would be easy to show this in portal (webdynpro abap) application but I am not sure if this can be achieved in PR05 screen too without any modification - which is obviously NOT recommended.
I hope technical experts will not hate me for having a go at this but basically, I believe you can do this by saving your custom status in a custom table, write your code/logic/condition in the method and then have this method call in your workflow task after the after the approval task and pass the values for your custom status in the WF container.
There is also a document by Sally Redmond (see link below) which talks about where and how to view the status of the filled by workflow.
http://wiki.sdn.sap.com/wiki/display/ERPHCM/Workflow+Customizings+in+Travel+Management
I hope other will shed some light on this topic from a technical/workflow perspective.
Ankur