It’s not possible, you need to create it as a custom field or in your survey flow.

Thank you ​@vgayraud Vincent. Can I add/combine custom formula fields? Basically I want to reuse the custom field.

Sorry I wasn’t clear,

I meant as custom fields in the field editor module, not as new variables in Stats iQ. Custom fields allow for manuel, bucketing, formula and combine variables.

See https://www.qualtrics.com/support/survey-platform/data-and-analysis-module/data/add-new-fields/manual-variables/ and following sections for more info.