Ok, it's weird to answer my own post, yet I learned how to do it and in case someone else is just starting up, it might be helpful:
import the 60p footage into premiere, rightclick onto it in your footage-list and got to modify > interpret footage. there you can enter your desired framerate (in my case 25fps) and click "ok". then right click on the footage again and choose "new sequence from clip" and now you have a 25fps-sequence with true slow motion footage.
thanks to Adobe ambassador Jon Barrie!
Are you in Europe?
What formats do you intend to deliver in?
Yes, I am in Europe and the intended format will be a H.264 PAL mp4-file with 25 fps in a resolution of 1080x720.
Any suggestions? :-)