The load combinations considered for regular/irregular building in sizing are with factor 1 for all load cases. Can this factor be an input?

Factor can’t be an input, as for sizing of footing/Pile-cap different factors are to be considered for dead, live and other load cases. However, user can create his own load combination templates by using ‘Add Load Combination’ option and creating a new template. User also can add or remove load combinations as per requirements. For more information refer Topic “Load and Load combinations” from help Content.