As the title says, again this is a vague issue I kept bumping in to as I browsed the forums, some people asking abotu it or saying it's doable but no hard facts to support it... so...?
Yes, just go to the Multiplayer Keep - and create your mp party there. Characters are automatically set to lvl 18, and you'll have to re-equpip them - you can't use a stored sp build sadly.
You may need to drag the table to see your inactive chars (to the left of your active party clipboard).
You don't HAVE to drag it: there are little arrow buttons at the far bottom to direct you this way. But dragging works.