Doesn't look like it.
Win2K service packs let you Slipstream the updated files from the service pack onto a copy of the i386 folder. So, you can copy the i386 folder from the Win2KPro cd into a folder on your network, then run the Slipstream command pointing to the i386 folder, and it will upgrade it to the service pack level. It is kinda cool, actually!
I was hoping you could Slipstream a RIS image, but I found this Technet article:
Slipstream Switch for Windows 2000 SP1/SP2 Update.exe DoesNot Work with RIS Server Images (Q258868:
http://support.microsoft.com/default.aspx?scid=kb;EN-US;q258868
It says that you can't slipstream a CD-based RIS image. You have to slipstream prior to making the RIS image.
So, to answer your question,no, I don't believe you can upgrade the SP level of your existing RIS image. Looks like you will just have to make a new RIS image with the proper SP level.