Well without it, I think you are stuck using the Character Map program to copy and paste special symbols. Most of the shortcuts that work in word processing apps don't seem to work in Excel. I'll give you the tweak anyway and you can decide whether to use it!
1. Open the registry editor (Start-Run, type 'Regedit' without the quotes and press Enter)
2. Navigate to (and select) HKEY_CURRENT_USER\Control Panel\Input Method and you will probably see two keys in the right hand pane ((Default) and 'Show status')
3. Choose Edit-New-String Value from the menu, name the key EnableHexNumpad, then double click it and change the Value to 1.
Now in Excel you should be able to hold down Alt and type + on the number keypad followed by 215b and then release Alt.