Off hand, I have to say No. If you have it set up as a Number field, then numbers are all you are allowed to enter. I could be wrong, just given my opinion as I do not know of a way to do that. I am sure that are work arounds for it.
Do you use this field for any calculations? I often use text fields in replace of number fields because I know the field is there for visual reasons and no calculations are needed on it. Does this field have to be a number field ?