Ah - I see what you mean - there is only a name field rather than separate firstname / surname fields.
And yes - creating separate firstname and surname fields using Joomla's custom fields are going to be much more complicated to access then a column in the users table.
If it were me, I would either:
a. Use Fabrik calculated fields to extract the firstname and surname as the first and last words from the Joomla user name field; or
b. create an SQL View on the Joomla users table that does the same.
Ok - handling names like F. Scott Fitzgerald Jr. would require more complex logic, but for Frank J. Sinatra it would work just fine.