I am modelling an adjustable HSS structure in STAAD and am wondering about what to do at the points where the HSS members will be connected.  Should I model it as an extra thick HSS? If so does anyone have any tips?  Do I have to create a unique steel structure?  For visualization purposes there will be a HSS 7x7 as the base and the adjustable "slip" portion will be 8x8.

So just so I understand it, are you slipping the 8x8 over the 7x7 by some amount, and requiring the "joint" between the two to only work through friction and direct bearing of the one tube on the other?

Next question, will the loading cause the joint to slide open or closed? i.e. will there be tension/compression in these members parallel to their long axis?

Assuming no tension/compression (and I understand you correctly), and assuming enough overlap to develop moment, I would design the 2 pieces as one using the smaller section properties.  If you can develop a moment at the joint, then I would fix the connection between the two pieces, or even model it as only one piece.  You could model it as two pieces, rigidly connected, with the correct properties for each piece.  I don't think it would really matter too much which way you went here.  

But I would not count on Staad to accurately model deflection, since the two pieces would need to deflect relative to each other, before they would bear on each other to develop any moment.  And while loaded, I think there would be too much friction for any sliding to occur.

Thank you for your post.  Only thing I didn't mention was that the two different HSS pieces would be held together at varying heights by through bolts.  Part of my analysis involves deciding how many bolts I would need as the pedestal structure is adjusted upward. As of right now the structure is designed with bolts at every six inches up to 3'6".  The 7x7 piece is welded to a base plate and the 8x8 piece is the adjustable component which will be bolted as desired.  I am trying to use STAAD to define the forces that the bolts will take if a weight is loaded on top of the structure.

ok, i understand what you are trying to do.  however, i don't think you will easily find a way to get staad to give you bolt shear forces.

I think I would model it with a node at the centroid of the bolt line, with the two pieces rigidly fixed together.  Then I would take the moment, shear, and axial forces at that node to figure out my bolt forces.  In the absence of any significant axial forces, I think bolt shear will be fairly negligle, as you have a long enough overlap to develop a significant moment without any connections between the two pieces.  But you could ignore this, and decouple the moment over the 7" width of the section, to figure out bolt shear, assuming your bolts pass through the tension/compression flanges.  If they pass through the webs, they would not contribute significantly to the moment capacity.  Then take whatever axial forces are present, and add them to the bolt shear due to the moment.