calculated field to generate user initials

When we create orders in CRM, they are sent over to GP. When they go over to GP they are printed as invoices and we need to know who to hand them to. I would like the initials of the user who created the order to be sent over.

I would therefore like a calculated field on an order entity that takes the created by field and turns "John Smith" into "JS" and "Adam Jones" into AJ etc. I can then use these initials in my mapping over to GP. Taking the full name uses too many characters in GP hence my need for just initials.

Thanks