You can use a dropdown menu with pre-filled values with the currently available asset custom fields. This can be set up under Configuration\Asset Pages, scroll down to Asset Custom Fields and use the type 'combobox'. You can add values to the Values column. Changes to these values on this page aren't reflected on asset pages where values have already been selected though unfortunately, a new edit would be necessary on the asset page.
It's currently not possible in Lansweeper to add your own custom tables. We also cannot provide support for any database modifications you would make, as these could potentially undermine the integrity of your installation.