Mislav Kov has an informative post on generating sample data for different tables using custom functions, and he went the extra mile with a demo file and a video to help you out.
Kov started with a Modular FileMaker solution and made it better:
There's also a custom function in there that selects a random value from a value list that you specify.
Now I've gotten in the habit of creating a script for each entity (table) that I have to create test data for. The script fills in all of the fields using the custom functions. I place a button to call the script on the layouts where I need it for testing, and I use the FileMaker 13 "Hide object when" feature to hide the button when the user is not a full access user.
Setting up the script takes a few minutes, but it's well worth it, because it then becomes quick and easy to create realistic test data.
Give it try...
Source: Test Data Generator