Well, first of all send as who? you want to send as a shared mailbox another user? what is it you are trying to accomplish? this should help..
1. On an Exchange computer, click Start, point to Programs, point to Microsoft Exchange, and then click Active Directory Users and Computers.
2. On the View menu, click to select Advanced Features.
3. Expand Users, right-click the MailboxOwner object where you want to grant the permission, and then click Properties.
4. Click the Security tab, and then click Advanced.
5. In the Access Control Settings for MailboxOwner dialog box, click Add.
6. In the Select User, Computer, or Group dialog box, click the user account or the group that you want to grant “Send as” permissions to, and then click OK.
7. In the Permission Entry for MailboxOwner dialog box, click This Object Only in the Apply onto list.
8. In the Permissions list, locate Send As, and then click to select the Allow check box.
9. Click OK three times to close the dialog boxes.