Custom fields are already supported.
In "WEBSITES" -> select that blog -> add custom fields in "Custom fields" tab.
Edit:
The field name must be taken from HTML.
In this screenshot:
http://prntscr.com/7k9y1fThose 3 fields which have "iframe" inside in HTML are like this:
Code:
<textarea name="embed_pelicula" style="width: 95%;"></textarea>
...
<textarea name="embed_pelicula2" style="width: 95%;"></textarea>
...
<textarea name="embed_pelicula3" style="width: 95%;"></textarea>
So the fields names are:
embed_pelicula
embed_pelicula2
embed_pelicula3
Type: textarea (for all three).
You need:
1. Add custom fields which I specified above.
2. Remove the user.
3. Add the user again in order to re-fetch all posts correctly.