You can't do this with the default snippets unfortunately, you just have to overtype with the value you want.
You could try my completions
attached. When the values are highlighted press 'a' and it will write 'absolute'. If you backspace and type 's' it will replace it with 'static'. Then press TAB to move on.
Unzip and copy the file to your Packages/User folder - you can rename it, but keep the extension. You may also want to temporarily move the default CSS snippets-folders out of the CSS folder, so that they don't get in the way.
There are a number of other abbreviations included, such as for colour names. Let me know if you need some guidance.
Alternatively, you could look at the ZenCoding plugin. Andy.
"I'm here to save your life. But if I'm going to do that, I'll need total uninanonynymity." Me Myself & Irene.