Really easy Jeroen,
the pist is working but the lift is not, you need only the transport to the pist, may be you take the hiking route starting at the Hut Mussala, but be careful.
Or you hike there from Borovez needs only a hour....
Sometimes its possible to go up with a pistemachine/pisteboelie. When they are finished the 3 pistes infront of rila they sometimes bring people from top 4 chairman lift to top Sitnyakovo. A friend off mine tried two weeks ago but the pistes where not preperated and verry bad.