Dear Igor,
SAM's CSP models calculate the energy required by a gas-fired backup boiler, and allow you to specify the timing of the backup boiler's "fossil fill fraction" and operation schedule so that you can choose to operate the backup boiler during certain times of day and year. Although you cannot explicitly specify the boiler's capacity, on the Thermal Storage page, you can specify the fossil fill fraction as a fraction of the power cycle capacity for each of up to nine periods. So, for a system with a 100 MWe nameplate power cycle capacity, if you specify a fossil fill fraction of 0.05 for a given period, the backup boiler will supply the equivalent of up to 20 MWe of heat to the HTF in that period.
Best regards,
Paul.