There is now an opt in beta for Killing Floor that will allow modders to upload their content to Steam Workshop before it is made available for all players.
To be a part of this modders will need to join the KF Workshop group found here: http://steamcommunity.com/groups/KFWorkshop
Further instructions on how to upload their content can be found in that group and the Wiki.
WARNING: THIS BUILD IS NOT COMPATIBLE WITH THE LIVE BUILD OF THE GAME. YOU WILL NOT BE ABLE TO JOIN SERVERS.