As firefly uses the BIP39 mnemonic phrases as "seed", there can be more than one wallet attached to (derived from) one mnemonic phrase. However, it is not supported to migrate an existing Trinity seed into a secondary wallet inside a firefly account. Also, it is currently not possible to log into more than one account (and see the balances) at once.
So, at first, you will have to migrate all old seeds individually, resulting in multiple new Firefly "accounts".
After that, if you want to, you can create more wallets in one of the accounts and transfer the IOTA to them from your other (temporary) accounts.
Each of these wallets will use different IOTA addresses, so it is not possible to derive that they all belong to one person when looking at e.g. a Tangle explorer.
But you should keep in mind, that all these wallets are protected by a single mnemonic phrase, so anybody who has access to this phrase will be able to spend all your IOTA from all the attached wallets. So, if your goal of having multiple seeds was to allow others to access the IOTA in one of your seeds and not the others, you will have to stay with multiple accounts and check them separately.