It is correct that the combined footing job type does not allow one to define grade beams connecting the various footings.
However in STAAD Foundation Advanced ( SFA ) there are ways to handle this type of modeling.
To begin with, if your intention behind modeling the grade beams is to get additional resistance to sliding against lateral loads, what you can do is, account for the additional sliding resistance by adjusting the factor of safety against sliding from within the combined footing module. That way you are not modeling the grade beams but the sliding checks would not show as failed in your calculation sheet due to the lower factor of safety input.
Alternately if you are looking for an analysis of the whole system with the ribbed footings connected by grade beams, you may define the foundation job type as mat. This would allow you to model the individual combined footings using mats and you can then use physical beams to model the ribbed beams and the grade beams ( use the Physical Beam table within Mat Foundation job to model these beams ). The combined system can then be analyzed using a full fledged FE analysis in SFA.