Generate a Unique Numeric Code whenever a customer is Created?

no problem.  You are over thinking this, assuming that you just need a sequential number beginning with B.

The formula text formula would be

"B" & totext([Record ID#])

Now, you will need to think though how you want to start the numbering since you may not want to have the first Customer be Customer # B1.

So one choice is to import say 999 dummy records and hence start off at B1000.

Or else perhaps

"B" & right("0000" & totext([Record ID#]),5)

so that would read B00001.

assuming that you would never have more than 9999 Customers.