Most citation management programs provide ways to convert citations into common formats for transferring into other programs (i.e. from EndNote to Refworks, Zotero to EndNote, Refworks to Zotero, or Refworks to Mendeley). Below are some links to instructions for doing this.
If you are using another manager and want to switch to RefWorks.
Zotero makes it very easy to switch managers. The instructions below are for EndNote, but they easily translate to other systems.
At a most basic level the process is: 1) Export citations as a file in some format from your starting application. 2) Import that file into your new application.