How do I 'convert' letters to numbers?

I'm writing a program that converts regular bank account numbers to IBAN numbers. Part of this process means I need to convert the letters (in fields like Bank-ID, Branch-ID and ISO-Country-CODE) into the numbers corresponding with their positions in the alphabet + 9. So A=10, B=11, Z=35.

So basically, the following Bank-ID: "LOYD" would become "21243413".

I need to put these numbers in a string later so I make calculations with it.

Now, if the input bank-ID would always be of equal length and always letters, I'd put this among the the rest of my working storage:

LETTERS    PIC X(36) VALUE '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ'.

And then I'd write this:

INITIALIZE BANK-CODE-NR           PERFORM VARYING LOOP-COUNTR FROM 1 BY 1 UNTIL                           LOOP-COUNTR > 4           INSPECT         LETTERS           TALLYING        BANK-CODE-NR(LOOP-COUNTER)  FOR                           CHARACTERS BEFORE INITIAL BANK-CD IN                           COPYBOOK(LOOP-COUNTR:1)           END-PERFORM

But in this case, the bank ID may be four positions long, or even longer. Plus, I'm working with PIC N fields, which means the INSPECT doesn't work.

EDITED FOR EXTRA INFO

I'm working with a copybook that has nationals in it.It is structured as follows:

     01 Input-IBAN.          05 COUNTRY-CODE               PIC N(02).          05 BANK-ID                    PIC N(10).          05 BRANCH-ID                  PIC N(10).          05 ACCOUNT-NR                 PIC N(28).           05 Output-IBAN                PIC N(34).

It's a called program.