I hate to be a wet blanket but in some areas of the county, all old people who are friends of the family becomes Uncles and Aunts. Are you sure that they are biological Aunt and Uncle?
If so the unknown parent is the best way. You could also tie the uncle or the aunt to the person in you tree and use the "Related" tag in the Relationship window.
I believe the way you would do it is attach the Uncle to your father as your father's child. Go into the relationship window and change the relationship to "Related" Then you could attach the Aunt to the Uncle as a spouse.
This way would create some chunkiness in the tree but it would avoid entering another unknown person in your database. When you found the Uncle and your father's parent, you could then add parent, detach the Uncle from your father and attach the Uncle to the father.
In those Cases where friends are important but not biologically related, I research their family and formally write a report of my findings. I include stories or other information about why the person is important. I then keep the report in an area of my disk reserved for those friend but unrelated individuals. As I learn more about that friend I will update my report.
I suppose you could attach the report to your father and call it your father's friend. That way it would be noted in your database but not distort the family reports and charts.