Angel, I believe this is not possible, you can at most indicate the field names, example:
: AddDbf ("CUSTOMER", {"Customer-> number", "Customer-> description"))
Now, to solve this your need, you could use array:
cCampos := 'numeronotaxxx,N;descricaoxxx,C'
oFastReport:addArray( 'PEDIDOS' , aVendas , cCampos )