How to create Subtype from Standard BUSNO

I hv created Customised Busnumber from standard Busno " BUS1065 "

Standard Busnumber : BUS1065

Customised Busno : ZBUS1065

Now, Query is that how to create Subtype for that , how to do it so that we can used that Customised Bus number in the workflow .

Give the Steps for that one .

Please Suugest me the answer .

Regrads,

Sandeep Jadhav