Its not easily customizable. May be for a reason. If you absolutely must, here is one way. Disclaimer ofcourse is this is just an approach and not the official recommendation.
Overlay /libs/dam/gui/components/admin/schemaforms/formbuilder at /apps/dam/gui/components/admin/schemaforms/formbuilder. Then add your field at /apps/dam/gui/components/admin/schemaforms/formbuilder/formfields/autocompletefield. In autocomplete.jsp ,please remember to add <input type="hidden" name="./items/<%= key %>/metaType" value="autocomplete"> . Basically a unique metatype. Add that new field to /apps/dam/gui/components/admin/schemaforms/formbuilder/builditems.jsp how it has been done for other fields. Overlay /apps/dam/gui/components/admin/schemaforms/formbuilder/view.jsp and update String formFieldsPath ="dam/gui/components/admin/schemaforms/formbuilder/formfields";. This was originally formFieldsPath ="/libs/dam/gui/components/admin/schemaforms/formbuilder/formfields"; and only seached for fields under this path in libs. Now you should be able to see the new field. I did a sample at https://www.dropbox.com/s/3b6ow6vgx58l192/autocomplete-4.zip?dl=0