You cannot CHAIN to a file using a key where there is not access path built over that key.
You can have a LF with multiple fields as a key. Like: COMPANY, WHAREHOUSE, DEPT, SALESMAN, PART# and then only use part of the key to chain, COMPANY, WHAREHOUSE, DEPT.
But if you only have 3 fields in the access path, you cannot chain with 5 fields.
You can use SQL to select record(s) with any number of fields.