I'm not to sure this will answer your question but I believe it is useful information. I have never modeled anchor rods in STAAD before; I have always had a support where an anchor rod would be and I would than take that reaction and design the rod/baseplate/shear lug/etc by hand or spreadsheet I have made (using AISC design guide 1).
I'm not sure what is meant by "create a void in the solid", so I'm not able to answer that.
I don't believe you can make a solid a compression only member. Instead of using a solid to fill a small gap; you could essentially use some type of member/beam, which you could specify as a 'compression only' member.
If you are using a solid element for the foundation design one thing to keep in mind is that solid elements do NOT transfer moments and you are not able to assign a soil spring to a solid element (if the foundation is on soil).