In the report designer, select the Field List for collectionDataSource1
Right click any field in the field list and select
Add Calculated Field
Right click any field in collection data source and select Edit Calculated Fields
Edit the properties for your calculated field.
Make sure to set the Data Source to be collectionDataSource1
Click Into the Expression Editor to set the expression for the calculated field
Now you should be able to use your calculated field as a normal field on your report