Q. Can I install sample data after I have installed Joomla ?

A. Yes.

Solution:

• Unzip the 1.5.17 Full Package on your PC
• Browse to the joomla.sql file (Joomla_x.xx/installation/sql/mysql/sample_data.sql)
• Open sample_data.sql
• Search #__ replace with jos_(or your table prefix)
  N.B. #__ is two underscores.
• Save in plain text format.
• Open your Joomla's Database with phpMyAdmin
• Import your edited sample_data.sql

Further reading: joomla install sample data Unzip Full Package Beginners

Install mysql Test (Sample) Data Generators