Hello,

it's easy you have 2 choices:

1. Use the method that you did now, create a dataset including only the data source with the custom query. At that point in the control panel you have a dropdown to select the dataset and all links will refresh for that dataset, including the RSS feed link

2 Don't use datasets, simply use the options of each data source to exclude/include it in the RSS feed. So you can have the regular K2 data source excluded and the custom K2 data source included in the RSS feed

Hope to have explained well.