OK, I just try to make the situation clear...

You receive 8 ASCII-Characters "3", "4", "5", "6", ....
Then you want to pack 2 digits in 1 Byte and write them to the eeprom.

Yust try:
Chars Var byte[8] ;here are the 8 digits
For I=0 to 3 ;generate 4 bytes
;cut off unused bits and pack 2 of them
Dummy=(Chars[I<<1]<<4)|(Chars[(I<<1)+1] & $F)
Write I,Dummy
next I

(Very quick and quite dirty!)