Incorporating known ciphertext into new message
Not sure if this fits your requirement, but, here goes.

Using the homomorphic properties of ElGamal, you could turn $E_{pk}(b_0),E_{pk}(b_1),E_{pk}(b_2),E_{pk}(b_3),E_{pk}(b_4),\cdots,E_{pk}(b_n)$ (where $b_i\in\{0,1\}$, i.e., individual bits), into $E_{pk}(b_0||b_1||\cdots||b_n)$ (where $||$ is concatenation). This is done by using homomorphic scalar multiplication and addition (multiply by 2 then add the next bit, repeat as necessary).

This could be generalized to larger messages (not just individual bits) if Trent is told the lengths of the messages.

Any cipher that is homomorphic with respect to addition could be used.