You can write a script for this:
# loop through all post
GET https://lemm.ee/api/v3/post/list?limit=50&sort=New&community_name=fedigrow&page=*page*
# take `post.ap_id` for each item
# then fetch it over activitypub on the other instance
GET https://lemmy.ml/api/v3/resolve_object?q=*urlencode post.ap_id*
# same thing for comments
GET https://lemm.ee/api/v3/comment/list?limit=50&sort=New&community_name=fedigrow&page=*page*
# again fetch
GET https://lemmy.ml/api/v3/resolve_object?q=*urlencode comment.ap_id*
