Porkbun definitely lets you do that, they allow you to add an "external" domain which runs on their DNS (actually Cloudflare) but isn't registered through them,

Porkbun definitely lets you do that, they allow you to add an "external" domain which runs on their DNS (actually Cloudflare) but isn't registered through them, then you can seamlessly transfer the domain in to Porkbun later. IIRC Spaceship (Namecheaps soft-relaunch) lets you set up your DNS records as soon as you initiate a transfer to them, before it completes, so the records are ready as soon as the nameservers switch over. Not sure about the original Namecheap, I haven't used them for a while.