To customize dropdown lists in workflow you can use a CommServDBQuery activity to retrieve the list of items the user can pick from and then use a PopupInput activity for allowing the user to pick from the dropdown. Since you are new to workflows, whenever you use interactive activities like PopupInput and InformationalMessage, make sure to set the "Start interactive session" to true on the General tab of the Workflow properties. Also after the last interactive activity, you'll want to use the EndUserSession activity before you get into the actual meat of the workflow otherwise you may get timeout errors on the user's client.
I attached a simple workflow that demostrates this process using the following query to get the data to populate the dropdown
where idataagent = 'SQL Server'
To import the attached workflow, just rename the file to PickClientExample.xml and then you can use the following BOL link to import the file: