You can use web api for that for more info: https://checkmk.de/cms_web_api_references.html#Hosttagbefehle
- you have to use get to old site, you will get all tags
- use get to new site you will get configuration_hash key
- copy the output from the step 1 and paste it in the set method with the configuration_key of step 2
if you have more than the default tags on new site you have to merge them with the output of the step 1, if not you can just copy and paste