Php code to populate the 'allowed values list' of the List_field

When messing around with the drupal 7 core fields, I ran into some problems.
I would like to add a list-field to comments. This list-field needs to be populated through the 'allowed list values'-box.
With the php-filter module enabled, it looks like it wont give me the choice to use php code to create values.

The basic idea is:

when adding content -> 2 text_fields filled in by the author.
Comments fields -> list_field populated with the two values from the two text fields.

So I need to fetch the values (from the db) of those fields and put it in the list.

Is this possible?

Thx in advance.