I know that it is possible to overclock an igpu on a z170/270 chipset and a K Prozessor and some notebooks but is it possible to also underclock the igpu on this prozessors.
I know in idle the igpu go down to 350 mhz itself but i wantet to run it with only 1000 MHZ to test something.
Yes, it is possible to overclock the graphics frequency, although not recommended.
Overclocking and downclocking are steps out of scope of support. My suggestion would be visiting different forums on Internet for other peers that may provide instructions for this type of inquiry.
Additional information here