Go to Data Search and open the data table whose data you want to merge with your custom table.

Click the gear icon on the toolbar and select New → Custom table → Add to existing union.

Choose the custom table you want to merge with the current query.

Those column names of the custom table matching the ones in the query will be automatically merged. The rest of them will appear as not assigned. On the right side, in the Current query fields area, you can select specific fields from the query and drag them to the custom table, in the Query field value column. Note that the data type of both fields must be the same.
In the following example, we are merging the serverIp column in our query with the srcIp column of the custom table. See that the action and dstPort are automatically assigned with the query columns with the same name.

Optionally, you can create more columns selecting New field. Enter the new field name and then drag the required field from the Current query fields area.
See below how we add a new field and drag the virusID column from our query to include those events in the union table. You must set a name for the new field and change the data type if needed.

Click Save once you finish. Now go to the Data Search area and open your custom table. It should contain the data merged from the query.

Create a union table