Cheryl, welcome to the forum.
In simple terms a field cannot be both character and numeric.
However you could consider creating the extra calculated field as numeric and then only populating it with a value which can be numeric - so any alpha only values would perhaps need to be zero? (Null may give you other problems at some point since the only reason I can think of for requiring a numeric field is to perform some mathematical calcualtion later or for formatting with $ or thousands, etc.)
So something like
shoudl be close to what you would need for that (I have not tested it but it looks about right provided it is for a numeric field.)