Subject Field computed by a stored proc

How can I create a field computed by a stored proc ?

Create table test (
Test_id Integer not null,
Name VarChar(80),
FirstName VarChar(80),
Company VarChar(80),
FullName computed by ((select fullname from sp_get_fullname(test_id, name,
first_name, company)))

Firebird refuses to compile that statement.

Can you help me ?

Thank you very much, regards,

Pierre Y.