Hi Attila,
Follow the Installation Guide on the link below which will outline how to accomplish this installation properly:
https://support.symantec.com/content/unifiedweb/en_US/article.DOC9257.html
If the software doesn't allow you to install the Detection Server on the already installed Enforce Server, there is a reason for this and I doubt you will get around this. Should you do so, you'll be running an unsupported configuration.
Thanks!