how can the sbuf register be told to send a string that is received?
First of all, you do understand that SBUF is actually two separate registers, don't you?
There is a read-only SBUF, from which you read received bytes; There is a write-only SBUF, to which you write bytes to be transmitted.
Second, you must realise that these are just single-byte registers - therefore they cannot handle strings directly.
To handle strings, you need to write your own functions to handle the strings one byte at a time - or use the supplied library functions.