I am not sure which lenses are available for the A100, but I should think you have a couple of choices.
A dedicated Macro lens at around 100mm will give excelent results and 1:1 reproduction ( true macro) and could be used for portraits but you may well have to soften the results for portraiture, as they will show every single blemish!
Another choice would be something like a 70-300mm range with a macro setting, allthough this would not be a true macro at a 1:2 reproduction, but this would be a more versatile lens.
You may also be able to use extension tubes or bellows with your current lenses or even a lens reversing ring, all these will increase your options for close ups.
It all depends what is important to you, if you want really good macro shots, get a dedicated macro lens.
I am sure if you browse the various forums/manufacturers on the web you will get some more ideas.
Hope this helps.
Steve
Hi. I have been using my new sony A 100 which I purchased a while ago now. The lenses are not cut out for what I want to capture. i.e. Very small insects, Leaves, Plants, Flowers and the like.
I need a lens that is designed specifically for macro although it would be good to have one which could also be used for other work too. Something versatile. Any suggestions??
Thanks.

You don't take a photograph, you make it. - Ansel Adams