Musical Theatre-Style Bar Numbers

[Calvin Ransom]

Hello everyone, 

I’m transposing a musical theatre book and sometimes the bar numbers contain letters then go back to normal numbering.  

Here is an example for the Bar Numbering that I am looking for: 

1 2 3 3a 3b 3c 4 5 6

Does anyone know how to do this? 

Regards, 

Calvin Ransom