Custom field, add default value with text input field (like entering category name / brand)

Hi,
I have added an extra field, named 'style'.
This field is not in the feed, but it depends on the merchant.

When registering a feed, the category or brand field can be selected from dropdown or entered in text field.
Is there a way I can set the value of the 'style' field to a default value.

I tried to do it with a search raplace filter.
Wanted to replace the empty value of the price field with my own text.
I can't figure out how to search for empty value with the filter..

thanks in advance
Bas