No, Qlik Sense is not a spreadsheet application. You can't add arbitrary references to other pivot table cells. You can create an extra field in the load script that links cost categories with the group names. You'd create an extra table that looks like:
ReportGroup CostCategory Excess mileage allowance Excess mileage allowance External Training External Training Gratuity Gratuity Long service awards Long service awards Occupational Health Occupational Health Other allowances Other allowances Recruitment & relocation Recruitment & relocation Redundancy payments Redundancy payments Staff subsistence Staff subsistence Staff welfare Staff welfare EMPLOYEE COSTS OTHER Excess mileage allowance EMPLOYEE COSTS OTHER External Training EMPLOYEE COSTS OTHER Gratuity EMPLOYEE COSTS OTHER Long service awards EMPLOYEE COSTS OTHER Occupational Health EMPLOYEE COSTS OTHER Other allowances EMPLOYEE COSTS OTHER Recruitment & relocation EMPLOYEE COSTS OTHER Redundancy payments EMPLOYEE COSTS OTHER Staff subsistence EMPLOYEE COSTS OTHER Staff welfare Callout - salaries Callout - salaries Employers NI contr salary Employers NI contr salary Employers pens contr sal. Employers pens contr sal. Overtime - salaries Overtime - salaries Salaries Salaries Source Bonus Payments Source Bonus Payments Standby - salaries Standby - salaries SALARIES Callout - salaries SALARIES Employers NI contr salary SALARIES Employers pens contr sal. SALARIES Overtime - salaries SALARIES Salaries SALARIES Source Bonus Payments SALARIES Standby - salaries
You can then use the ReportGroup field as dimension instead of the original CostCategory (or whatever its actual name is).