Finally I used this workaround:
-I implemented a step calling Unlocker.exe (
http://www.emptyloop.com/unlocker/) in command line with -s -o parameters.Normally this program should unlock the related folder but in my case it does not happen but it writes in a log file the process id that keeps the folder locked.Log file is located in the same folder with Unlocker.exe
-I implemented a VBScript step that would read from the log the process id
-I implemented a kill process step for the process id deterrminated in previous step.