Hi,
You go with custom tool to create Tag instead modifying OOTB. but if you want to do it. Follow below steps -
1. overlay /libs/cq/tagging/gui/content/tags/createtag/jcr:content/body/items/form/items/wizard/items/createStep/items/fixedColumns/items/fixedColumn1/items and create Multifield like /apps/cq/tagging/gui/content/tags/createtag/jcr:content/body/items/form/items/wizard/items/createStep/items/fixedColumns/items/fixedColumn1/items/multifieldcollection/field/items/enterlinktext

2. Create your own servlet to create tags by extending com.day.cq.tagging.servlets.TagCommandServlet servlet
http://localhost:4502/system/console/servletresolver?url=%2Fbin%2Ftagcommand&method=POST
3. Change action at /libs/cq/tagging/gui/content/tags/createtag/jcr:content/body/items/form to point your custom servlet.
Thanks
Arun