Possible to setup MacBook as device like Ipod .... and sync to desktop computer?

Sort of, via Home Sharing. The analogue of "syncing" would be to connect to the Desktop library from the Air via the Home Share, then select "Items not in my library" in the "Show" menu, and then either select all and import, or select some of the items and import.

You'd have to transfer across playlist metadata by other means. Could walk you through that too, if you like.

and only want to move certain playlisted music to my Mac .... maybe 1k files

Can be done.

Start on the Air. Select the Desktop library. Click the wee disclosure triangle next to the name of the library on the desktop. You should see a "Playlist" entry below that. Click the wee disclosure triangle next to that. Select the playlist that you want to transfer content from. Select some or all of the songs, and click "Import".

Here's a screenshot showing that on my system: