If the value of the additional fields does not change, then you can use the Nutch's index-static plugin. It allows you to add a number of fields with their contents. You first need to enable it in nutch-site.xml. You then add the list of fields as shown below:
<property>
<name>index.static</name>
<value>indexed_by:solr,crawled_by:nutch-1.8,crawl_name:nutch</value>
<description>
Used by plugin index-static to adds fields with static data at indexing time.
You can specify a comma-separated list of fieldname:fieldcontent per Nutch job.
Each fieldcontent can have multiple values separated by space, e.g.,
field1:value1.1 value1.2 value1.3,field2:value2.1 value2.2 ...
It can be useful when collections can't be created by URL patterns,
like in subcollection, but on a job-basis.
</description>
</property>
If the value of these fields is not static and independent of indexed documents, then you will need to write a IndexingFilter plugin to do that. Have a look at the index-static plugin to know how implement yours.