There isn't a built in feature for this, but you can use field values to accomplish this by setting your Lookup Rules to a stored procedure with a parameter.  For example, if you want a lookup tp run in the second task of a process and you are adding Employee ID based on the ID number, you can add the employee ID to a specific field via workflow or a formula when another field value is met based on the task and let that be the parameter to trigger the lookup.