There is no such direct way to model the encased column in Staad. But still if you want them to be simulated in Staad then you can adopt some indirect trick.
For example you have a composite member where the Steel I-shaped section is encased inside the concrete column section then you need to determine the equivalent section with the same material property whose section properties like Ax or IZ or IY are close to the actual composite section.
But assuming that it is a column section which is supposed to carry the significant magnitude of axial force as compared to the flexible, you might want to calculate the equivalent section whose AX is very close to that of the actual section.
For example you have a composite member where the Steel I-shaped section is encased inside the concrete column section then you need to determine the equivalent section with the same material property whose section properties like Ax or IZ or IY are close to the actual composite section.
But assuming that it is a column section which is supposed to carry the significant magnitude of axial force as compared to the flexible, you might want to calculate the equivalent section whose AX is very close to that of the actual section.