Keeping an externally created field in solr.

[Alaak]

Hi,

I have an external program which changes a field for some websites 
within my Solr index. Nutch sets this field to a default value using a 
plugin on indexing a page. My problem now is that nutch resets the field 
for already indexed pages as well, when it updates those pages. Do I 
have any possibility to tell Nutch it should not touch that field if it 
already exists within the Solr Index?

Thanks and Regards