Within the menus, there are LOTS of ways to swap and save locations between Favourites, user-POI categories, Recent destinations, My location etc.
The two main places to achieve it are
1.in the "Manage POIs or "Manage Favourites" preferences/settings menus, and
2. on the Browse map page, where you can use "Find" to find just about anything, followed by the blue cursor button to Save it as something else.
(If you want to save anything to a user-POI category, you DO obviously need to have created the new category already)
Alternatively, if you have a long list of Favourites you want to convert to a user-POI category, you can do that too, using a utility such as POIEdit on the PC, to open the MapSettings.cfg file from your TomTom's map folder, and then to save the bits you want as an .ov2 file to go back onto the TomTom (in the map folder)