Yes you can but you'll need to install the DS Toolkit in order to make it work.DS Toolkit v1.7
You then need to start Dungeon Siege with either DSLOAMod (expansion pack) or DSMod (original game) and start a new game on the map you wish to play.
Then press the tilde key (~) which is to the left of the 1 key in order to bring down the console.
Type: PARTY LOAD CHARACTERNAME.dsparty where CHARACTERNAME is the name of your multiplayer character.
Press the tilde key again to close the console and save your game. Exit DSLOAMod (or DSMod) and start the retail exe. Now load the saved game.
It is extremely important to note that this method generally fails in the retail maps due to inherent bugs in them that prevents saved games from DSLOAMod (or DSMod) being loaded in the retail game.
However I have made a mod that fixes these bugs and also provides you with Veteran, Elite & Xtreme versions of the retail maps. After all even if you can get the above method working in the retail maps, it will get pretty boring playing them if your multiplayer character is of a high enough level, not to mention that experience received & item drops will be poor.
DSLOAMT will even give Elite players a pretty good challenge, especially if they attempt Xtreme mode. If you wish to try it it can be downloaded here;DSLOAMT v2.2beta