Had the same problem here. Tried to drag the violin of a single player to a violin of a section player I had created for that (it contained nothing) but Dorico wont let me. One can only drag as long as the section player is empty-handed.
I did this before but had to look it up again with Google.
I totally agree with adrien, this should be easier (best would be if there was a way to simply change a single player into a section player; Dorico can give a message if this is not possible because the single player holds more than one instrument).
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OK I accidentally had a Bass Clarinet section instead of a Bass Clarinet player. Trying to follow the directions above, I created a new Bass Clarinet player; but I could not figure out where to go in Dorico to do this:
“…expand the card in the Players panel in Setup mode belonging to the existing player, grab the instrument from the existing player, and drag it to the card of the new player, then delete the now empty-handed existing player.”
OK, I fumbled around 
and created a new blank player. I dragged him up just underneath the bass clarinet section. I expanded the bass clarinet section card and dragged the instrument to the new blank player. Then I deleted the section. That seemed to work. We will see. Thanks.
*This is not logical to me and I don’t know how the staff got assigned correctly, but it appears to be working,