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Could you please clarify a little bit more on your requirements. Basically if your system has API you can use it to create modules for Sitefinity which will let you manage data in your application from your Sitefinity back-end. Furthermore if you do not wish to manage data from Sitefinity and you just need to pull data from your application and re-use its controls you can wrap your controls in ascx files and add them to the Sitefinity toolbox. If the controls can be used on regular ASP.NET website they would also work in Sitefinity.
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