I'm not sure what you are asking. Here are a couple of KB articles that
describe this process, one for XP and the other for Windows 2000:
XP
http://support.microsoft.com/kb/314056/EN-US/
Windows 2000
http://support.microsoft.com/kb/250320/en-us
If your question is whether third party programs can make use of this
process, I don't know that kind of answer.
http://msdn.microsoft.com/msdnmag/issues/01/12/XPKernel/
discusses in greater detail what this critter does for a living, but
skimming it, I didn't spot a clear statement about whether or not a
third-party service can run under this process.
If your question is: Could some malware create a service and set it to
execute under one of the SVCHOST instances, without Windows Defender
noticing?
Here's what I read in the Help with regard to one of the real-time
protection agents:
---
Services and Drivers
Monitors services and drivers as they interact with Windows and your
programs. Because services and drivers perform essential computer functions
(such as allowing devices to work with your computer), they have access to
important software in the operating system. Spyware and other potentially
unwanted software can use services and drivers to gain access to your
computer or to try to run undetected on your computer like normal operating
system components.