Hi CloudenL and welcome to the forum.
Have you tried the ISALPHA() function? It checks for the first character in a specified string being alpha. See the Help for an example.
So, off the top of my head,
IF(ISALPHA(field)=1, field, str(val(field)))
should be close to what you require assuming the cell needs to be character. If you want the number as a number skip the str() bit BUT that will have other consequences you will need to consider.
If the characters are at the END of the string you could use something like ISALPHA(Right(field),1) and then include that as an alternative in your IF statement.
If the alphas could be in the middle of the 'number' things get more interesting. If they could be randomly located the challenge becomes quite exciting!
One approach to that would be to use LEN() to get the length of the string and compare that with the length of the result of the of using STRIP() to remove any alpha characters (or numerics I suppose) and if the length stays the same (or if you strip numerics, hits 0) then you know whether to retain it as it is or convert it to a VAL().
Which approach you prefer will depend on what sort of issues you may need to contend with. Indeed you can use a combination of these techniques to automatically highlight any records that seem to break any rules you expect to have for data formats within the incoming records. That in itself could be useful as an additional feature for your extract.