The only way to input alphabetic text is to have a string field. Though TECHNICALLY, if the input is to a text box on an unbound control then it can be any character you want as long as you don't specify an input mask.
As Paul states, if you are inputting to a numeric FIELD then you are stuck to a very few cases. You can input the letter E, a decimal point (period), and plus or minus signs if your field is SINGLE or DOUBLE (which are, indeed, numeric formats). You would do that to input numbers in scientific notation: snnnnnn.nnnnnEsnn But otherwise? If it is bound to a table, YOU are bound to the rules of the field.