>>>is possible to create a calculated field in a VFP table?
>>You need to create a view on the table and use that view
>
>Thank you.
>I need an index on computed field :(

You can index a view -

&& form load 
use viewname in 0 nodata
select viewname
index on fieldname tag xxx

&& later 
Viewparam1 = ...

=Requery('viewname')