There are some different scenarios to consider.
1) Is the external form already posting or re-posting to Eloqua? If yes, you have the form and integration details to use within Eloqua already and can likely drag it on to an Eloqua page or use the HTML details option if you have an HTML page.
2) Is the external form only posting to an internal database and not to Eloqua? If yes, you can use the "Integrate an External Form" option in the forms area of Eloqua. This is a quick way for you to enter the URL of an existing form page and have Eloqua set up a form for you that already exists.