Adding new document type in purchasing

When I try to add new document type in purchasing I can pick from a list tow options: Quotation and Request for Quotation. However, I need to create a type that I could associate approval limits groups to it and add it to position heirarch. The new type will be used to pass heirarchy approvers and their limit to another integrated system that will use this information. I don't neccessarly want to affect our current approval limits for exiting document types. Could you please advise how to enable this?