How can I add non numeric sms senders to my contacts (for custom alerts)

ADB should accept non-numeric contacts.

Download Terminal Emulator from Play Store, or hook up your phone to a computer with ADB drivers installed and USB debugging turned on.

Then enter:

$ am start -a android.intent.action.INSERT -t vnd.android.cursor.dir/contact -e name 'John Doe' -e phone 956-0987 <-- Replace with non-numeric

And/or:

$ am start -a android.intent.action.ACTION_INSERT -t ContactsContract.Contacts.CONTENT_TYPE -e ContactsContract.Intents.Insert.NAME John ContactsContract.Intents.Insert.PHONE 956-0987 <-- Replace

Both will still open up a "New Contact" window, but the information will be pre-entered.

If your Contacts app will not accept the data, you can actually solve your original problem using Tasker, a phone automation app, and setting up a profile Event -> Phone -> Received Text and playing whatever system ringtone, or personal ones saved in /sdcard/Ringtones by using Action -> Media -> Play Ringtone. Here's a screenshot of Tasker's UI:

Hope this helps!