Transfer site from WordPress.com to Multisite and maybe transfer again later

Not sure if this is beginner or advanced but here goes, two questions:

1-Have a client with a WordPress.com site and they can pay $119 to transfer to .org, I cannot find a way to do this other than export/import? Correct?

2-So if we import this site into our newly built multisite server we've essentially will need to do the same export if their site ever "outgrows" the multisite network

Maybe this doesn't happen and they will never outgrow the multisite?

We are used to having a single site DB and backup/restore of entire sites... is there anyway to do this for a single site inside of a network... sorry question 3