Hi,
Here are the steps that you need to follow:
1) Use a Dialog Workflow Component
2) Inside the Dialog Workflow Component put an External Dialog Step Component
Next the key thing is to configure this component with the Url of your external application, various paths, the data that you want to expose from your workflow project and also the data that will come back to the workflow from your external project.
Then Deploy your project, when you deploy this project, you will see other than the standard workflow webservice, you will also have a custom web service which can then be used to interact with the workflow which is paused at the External Dialog Step Component.
In your web application (asp.net project) create a Proxy for this web service and then you will be able to fetch data from the workflow and also put data back into the workflow. Once you have completed your interaction in the Custom Pages, just redirect the application to the Return URL that you received in this project.
Please feel free to ask more questions.