have you already read Easytrieve Variable declaration?
In COBOL, REDEFINES verb can be used to declare variables that share a same memory location.
In Easytrieve also, there is a similar functionality.
For example, the ALPHA and NUMERIC variables given below share the same 5 bytes.
WS-ALPHA W 5 A WS-NUMERIC WS-ALPHA 5 N
Since the starting position for WS-NUMERIC is given as WS-ALPHA, both of them start from the same memory location.
In case, if you want the same functionality in a File variable,
FILE INFILE IN-REC 1 80 A IN-ALPHA 1 5 A IN-NUMERIC 1 5 N FILLER 6 75 A