1
Create a dataset with custom variables
Create custom columns of the type 
Variable
in your dataset. All data from these columns would be available as variables in your agent.
2
Use variables in your agent
Use the variables in any of your nodes using the 
{{variable_name}}
syntax.
3
Test your agent
Trigger a test run and view the results.

Any value set in the agent while testing would be overridden by the value from the dataset.