Hmmm what do you mean by summed scale? If you are talking about transforming the missing values of a column in your dataset to be filled with the mean of the values that are not missing from that column, this is most suited to be done within analysis software.
If you are talking about obtaining a mean value for a single respondent based on the values of that respondent's other answers, you might check out Math Operations to save this value as an embedded data field. You could also create this mean for each respondent by using Formula Variables.
https://www.qualtrics.com/support/survey-platform/survey-module/editing-questions/piped-text/math-operations/
https://www.qualtrics.com/support/survey-platform/data-and-analysis-module/data/add-new-fields/formula-variable-creation/