@lydiaconwell If you know a pixelfed account, just go to their profile page in mastodon (you can put their pixelfed address in the search bar). From there on you can follow them and so on.
They're already in your timeline. You wouldn't notice much difference from a Mastodon perspective.
Just that when their address ends with 'pixelfed' in its domain name you know it's likely a pixelfed account. But there might be many more with vague domain names that might actually be a pixelfed instance without you realizing.
I took a glance at the federated timeline just then, with the option 'Media Only' turned on. I saw at least one domain that's pixelfed-related. So they're already federating with us and just looks like everybody else to us because of our Mastodon interfaces.