I must admit, I do not understand what you want. Mix between portal and site and template.
Please try to be more precise.
It is possible to use an existing BC site as your Partner Portal site. It requires steps seen in the video at BC Gurus "
Replacing Your Partner Website Template"
. So would the same logic hold that I could port over a muse site to my BC Partner Portal website.
You can certainly do this, however, please note that your old template site will be completely replaced with the new one which you will upload.
For further clarification, I suggest you post this query on Business Catalyst forum as well.