You cannot impersonate a user without knowing their username and password. A simple work around that I use under similar circumstances is to create a "Temporary Debugger" user in MS CRM. Assign the temp. user the same role as the user you want to impersonate. If there are certain records you want to work with then ask the system user to share the records with the temp user account.
You can disable the temp account after you have resolved the issue.
Hassan.
Hassan Hussain | http://hassanhussain.wordpress.com/