SAP support query.....Please Help

Hi,

I am a student at Lancaster University. I am currently conducting a dissertation regarding vendor support for organisations implementing Information Systems. I had question regarding SAP's view on third party support. After implementation of a SAP system are organisations allowed to choose a third party company (not recommended partners such as Siemens and HCL) for supporting their SAP system?

I would really appreciate to hear from you regarding this as I cannot seem to find this information on the website. Any help would be greatly appreciated.