>>>Naomi suggests preparing the formatted value in advance; in addition to that, you can also use the transform() function in the report expression itself ( I suspect this would be simpler). Just set the report expression to something like:
>>>
>>>

>>>transform(MyField, pcFormatExp)
>>>

>>>
>>Hilmar,
>>
>>This seems to work fine.
>
>I think I did that on some occasion.

I've updated my reply with the simple test I tried:

lcNumber = '1235678790'
lcFormat = '999,999,999'
?TRANSFORM(m.lcnumber, m.lcformat)