Match a Scratch username with your Snap username and you will have the option to transfer your Scratch projects to your Snap account, in case you get permabanned on Scratch but not on Snap, then you could edit your projects from snap.
The only scratch to snap converter is Snapinator, a tool that was not created by the snap team. The fact that it wasn't created by the snap team shows that they don't actually care enough to make one (but they do care enough to link this in the community site footer). And even then, it's not perfect, and most projects don't run the same.
Gathering all your projects would also be a hassle, and probably way too much load on the server, more than it's worth. It would first have to get a list of all your shared projects (it can't get any of your private projects), then for each project it has to download, and then convert it, and then save it to your snap account. Doing all that can take a long time, and I think they don't want the server to be bottlenecked converting hundreds of scratch projects to snap.
Probably the biggest reason they won't add this, is because they want you to explore what snap can do, be creative with snap. You shouldn't try to make projects created in scratch work in snap, since that's very difficult, and it's just not worth it. Go create new games in snap, using the unique features of snap instead.
Like the Snap! username has to match the Scratch username?
Usernames sadly cannot be changed without the workaround of creating a new account. There will be several problems:
One person could have two accounts, one on Scratch and one on Snap!. They could have the Snap! account for a long time, and yet unable to extract projects from Scratch as their dilemma to make the Snap! username distinct is final. They would have to use up time creating another account, and have to transfer the projects from the other Snap! account to their main one.
A Snap! account with a username that a Scratch account has would be worse. That person will get the ability to steal projects from the Scratch account and the person behind the Scratch account in Snap! will not get to. More awful if the Scratch account is a namesnipe (usernames with common keywords), which will have the chances of this issue higher.
Not only those, see above replies. Even if the user doesn't have issues of these, the suggestion doesn't seem plausible.
Snapinator doesn't convert a bunch of blocks for some reason. The last time I converted a project through Snapinator half the blocks weren't even converted