Changing a RT to create a stopover?

I have to fly from my home city in the US to a city in Europe without direct flights. The dates of my trip don't matter so much, but for reimbursement I need a RT ticket showing that I went to my destination city and back home. I was already planning on leaving 2 days before I need to arrive to adjust to the time zone, so I'd love to turn this into a mini-vacation. The best flight options I've found are Finnair with a layover in Helsinki, BA with a layover in London, KLM with a layover in Amsterdam, or Lufthansa with a layover in Munich. On any of those airlines, would I be able to book the ticket, and then change just the second flight of the outbound leg? I am fine paying any fare difference or change fees, I just can't get reimbursed if I purchase this as a multi-city itinerary.