For this there is currently only one solution, which is performing a manual import.
Hopefully you should be able to simply copy all your source code folders under the src folder of a newly created System Workbench project (as you will probably not need the ST libraries - you should already have them in your IAR source tree - create a project without any firmware). If you have dedicated “include” directories, you should probably place them in the “inc” folder of the created project; if you leave it in the source tree, they may still be referred to by relative paths.
Then you may have to suppress a few files from your copied files, like for example the stm32xxxx_startup.s file); when you have a conflict, you should probably suppress the IAR originated file (or just exclude it from build, to keep it as reference, by right-clicking it >> Properties >> C/C++ Build then check the “Exclude resource from build” checkbox).
You should also, of course, exclude the main.c file created in src by System Workbench (it does not contain any useful code anyway...)
You may also have to adjust a few build options in the “project >> Properties >> C/C++ General >> Paths and Symbols” tab (if you have include files that you do not place directly under the “inc” forlder in your project, or symbols that must be defined for your code to compile correctly) or “project >> Properties >> C/C++ Build >> Settings” tab (if you need to change some specific compile directives).
I hope this will help you; please keep us informed of your progress.