The footprint and symbol can only use numerical pin names, however you can still assign alphanumeric pin identifiers in the Part Editor.
In the Part you click on the "Pins" tab and in the column "Pin Name" enter the alias you want to use.
This is very useful when you have components that come in both BGA and QFP/QFN flavours and you may want to change between the two diffrent packages in a schematic without having you schematic get distorted.
For example where I have an ATxmega16A4-CU (BGA), ATxmega16A4-MH (QFN) and ATxmega16A4-AU (QFP) I've used this column to name the pins with an alias in the style:
<QFN Pin No>/<BGA Pin No>
ATxmega16A4-CU (BGA) Footprint Pin = 1 Pin Name = 43/A1
ATxmega16A4-MH (QFN) Footprint Pin = 43 Pin Name = 43/A1
The Pin Name column is also useful for Diodes, having pin names k or a, Transistors, having pin names b, c, e or FETS having pin names d, s, g since it means you can swap between parts having different styles of packages when the pin numbering on the footprint may be very different without affecting the connections on the schematic.