The simple answer is no. Although, I know that we can do it if your client/company is willing to shell loads of money to build this web service.
Nevertheless, your question is too vague to answer ... what external web application? Intranet or Internet? What version of SAP? Are any new dimension products installed? Do you plan to user NetWeaver? or Do you have SAP Portal up and running?
How are users currently authenticated in the current external application? How would you want to control authorizations? .... the list goes on...
