1:
You'll need to add a custom library field.
Go to Options -> Library & Folders -> Manage Library Fields
Click Add New Field, name it (For instance "Movie Genre" and click ok. You can modify the values in there if needed, but it's not really needed for custom genres.
Select the media to tag, go to the Action Window, click Tag, click the little list symbol on the uppre right corner of the action window and select Show all tags.
After you've done this, and tagged your media, you'll have to create a view to support this new library field. For Theater View, you'll have to enter Options -> Theater View and modify existing views or create new ones. For standard view, you just edit the current video views, or right click and create new views.
1/2:
Find your custom field, and enter the Genre you want. You can also select the normal [Genre] tag here, and select and write other values as you want. Those will not be updated again from the internet scraper, unless you manually select the files for online updates.
You can change almost all tags and their associated data this way. Even grayed out data can be manipulated if you create expressions. There are however some default library fields with default values that can not have other values than the predefined.