You cannot use the field as choice list with an custom table.

I propose to you to create this field as reference to your custom table and as referece qual to have:

javascript:RccFilterBsUserImpact();

In our sandbox, in the incident you will found the user impact field.

Script Include

function RccFilterBsUserImpact(){

    var bs = current.u_business_service.toString();

    var cat = current.category.toString();

    var resFilter = '';

    if (bs && cat){

          var m2m = new GlideRecord('u_m2m_user_impacts_services');

          m2m.addQuery('u_cmdb_ci_service', bs);

          m2m.addQuery('u_bs_user_impact.u_category', cat);

          m2m.query();

          while(m2m.next()){

                // += concatenate all retreive information

                resFilter += (m2m.u_bs_user_impact.sys_id + ',');

          }

    }

  resFilter = 'sys_idIN' + resFilter;

  return resFilter;

}