You might want to look into Apt pinning: https://help.ubuntu.com/community/PinningHowto
Summarizing, you pin your system to 8.10 but allow it to install a certain package(s) from the newer release.
Add the following (creating files as required)
deb http://your.favourite.mirror.net/ubuntu karmic main restricted universe
And then to /etc/apt/preferences:
Pin: release a=intrepid
Pin: release a=karmic
You may need to fiddle with this a little and install the jaunty version of samba first (especially as you're going across two releases here). You may also find that installing samba you're asked to install a lot of updated dependencies in which case you might have to consider rolling the box up. Most packages allow a range of versions and usually going forward one release is okay. All bets are off when you go further forward however.
You'll also need to be aware of the presence of this pin before you next do a complete upgrade of the box (removing or modifying it as apporpriate).