Once you calculate the section properties, you could use the General section properties type for the combined section and enter this into STAAD. This type does not allow you to use the AISC code check. But it would give you the stresses needed to evaluate the member.
However, you might just want to do this one by hand. The axial load and moments from the jib would be fairly simple to figure. Plus, you may want to add a plate to the other side of the W8x31.