A sample code for adding a custom field to a Solr index dynamically.

CustomDocumentBuilder.cs

public class CustomDocumentBuilder : SolrDocumentBuilder
{
    public CustomDocumentBuilder(IIndexable indexable, IProviderUpdateContext context) : base(indexable, context)
    {
    }

    protected override IEnumerable<IIndexableDataField> GetFieldsByItemList(IIndexable indexable)
    {
        var fields = base.GetFieldsByItemList(indexable);

        // Create a custom field
        // This field is added as "custom_field_t" in a index
        // See '/App_Config/Sitecore/ContentSearch/Sitecore.ContentSearch.Solr.DefaultIndexConfiguration.config'
        var field = new CustomField
        {
            Name = "Custom Field",
            TypeKey = "single-line text",
            FieldType = typeof(TextField),
            Value = "This value is added by the CustomDocumentBuilder",
            Id = Sitecore.Data.ID.NewID
        };
        
        return fields.Append(field);
    }
}

public class CustomField : IIndexableDataField
{
    public string Name { get; set; }

    public string TypeKey { get; set; }

    public Type FieldType { get; set; }

    public object Value { get; set; }

    public object Id { get; set; }
}

Foundation.Indexing.CustomDocumentBuilder.config

<?xml version="1.0" encoding="utf-8" ?>
<configuration xmlns:patch="http://www.sitecore.net/xmlconfig/" xmlns:role="http://www.sitecore.net/xmlconfig/role/" xmlns:search="http://www.sitecore.net/xmlconfig/search/">
  <sitecore role:require="Standalone or ContentManagement or ContentDelivery" search:require="solr">
    <contentSearch>
      <indexConfigurations>
        <defaultSolrIndexConfiguration type="Sitecore.ContentSearch.SolrProvider.SolrIndexConfiguration, Sitecore.ContentSearch.SolrProvider">
          <!-- Replace the SolrDocumentBuilder to the CustomDocumentBuilder -->
          <documentBuilderType>NamespaceTo.CustomDocumentBuilder, YourAssembly</documentBuilderType>
        </defaultSolrIndexConfiguration>
      </indexConfigurations>
    </contentSearch>
  </sitecore>
</configuration>