You may add the keypresses to an array using an index and then send the array over the serial port.

Not tested but you get the idea:

Code:
string    var byte[9]
index1    var byte

start:
For index1=0 to 8
@ READKEYPAD _myvar
	string[index1]=myvar
next index1

if myvar="#" then hserout [STR string\8]

goto start
Ioannis