Someone could send you their trustdb, but I'm not sure if you can merge them. (I've only backed up my trustdb so I wouldn't have to reset trust levels on the keys I trust.)
Regardless, it sounds like what you're after is transitivity: that if you trust A and A trusts B, then you trust B. That exists, but only to a single level.
When you mark that you trust a key, you're indicating how much you trust that key to verify other keys. (Usually, how much you trust the owner of that key to make certifications about the legitimacy of other keys.) The GNU Privacy Handbook describes the OpenPGP trust model and how trusting a key allows you to ascertain the validity of the keys in your keyring. At the bottom, there's a graph and example that's particularly helpful. Note that Alice trusts a handful of keys, and then that trust is used to ascertain the validity of keys signed by them.
So, in your original example, if you (fully) trust firstname.lastname@example.org and they've signed the key for email@example.com, then the pgp trust model considers firstname.lastname@example.org a valid key, but does not use signatures made by that key for any validity calculations. (Unless, of course, you've trusted that key as well.)
Note that marginal trust requires 2 trust paths to a key to consider it valid: this is useful for people who are not extremely careful about the keys they sign, or even if you're just paranoid and want 2 sources of truth.