Can one extend the nonce of ChaCha/Salsa20 by XORing the extra bits with the key?
Maybe. But your scheme hasn't been vetted by the community for its impact.

Better to use XSalsa20 or the related XChaCha20 as recommended by Bernstein himself: http://cr.yp.to/snuffle/xsalsa-20110204.pdf

In my opinion it was a fairly major faux pas that DJB originally chose short 64-bit nonces for Salsa20 and ChaCha20, especially given all the nonce-misuse attacks in the past. Storing a no-chance-to-repeat counter is actually really hard in practice. He then went on to make a "idiot-proof" crypto library that requires a user-supplied nonce for every "simple" box function, and yet doesn't include the nonce in the authenticated cipher text output.