There is no scale for import options, either in the Polymesh3d options or the Preference.
Short list of things you can do. Under Tool/Deformation there is a scale option. Drag it down (to the left making sure X, Y and Z are checked) by a factor of 100 several times (may take quite a few times) until it’s scaled down to a reasonable size. Then you can hit Store in the Tool/Preview window to ensure it’s stored at that relative size to it’s original size, then save it as a .Ztl.
Another thing you can do is setup any arbitrary model in zbrush, one of the z tools that come bundled such as the demo head or something. Export that as an .obj file. Then import it into your modelling software. Next, bring in the problem model and scale it relative to the imported zbrush model. Leave the scale and position of the zbrush model alone and bring down your model to the size you need using it as a visual or numeric reference for scale.
Once you’re done, export your object as an .obj then go back to zbrush. Go to the ztool you exported as an .obj and open it’s subtool menu. Next append the “Polymesh 3d” tool to that (looks like a star). Click the Polymesh 3d to make it active in the subtool menu, now go down to Tool/Import. Import the scaled down object and it will now show up in a workable scale.
You can then delete the reference ztool, save your model as a ztool and start work on it.