How to Move a Single WordPress Site into a Multisite

1st way to import single site into existing multisite

1. Crate New WordPress Multisite
2. Login to single WordPress site & exports Posts and Pages
3. Import into New WordPress Multisite.

 

2nd ways merge new single w0rdpress into multisite database method

Just export the posts & Posts meta tables. if required taxonomy taxonomy meta tables for categories & tags.  delete existing WordPress database tables.