16 and 10 byte keys use different constants, as described in the Salsa20 paper:

The diagonal constants are the same for every block, every nonce, and every
32-byte key. As an extra (non-recommended) option, Salsa20 can use a 16-byte
key, repeated to form a 32-byte key; in this case the diagonal constants change to
0x61707865, 0x3120646e, 0x79622d36, 0x6b206574. Salsa20 can also use a 10-
byte key, zero-padded to form a 16-byte key; in this case the diagonal constants
change to 0x61707865, 0x3120646e, 0x79622d30, 0x6b206574.

RustCrypto only supports the 32-byte key:

So it's not possible to "extend" the key without changing the constant.