Do you mean Slann as in leader of the Lizardmen, or as in the OLD Slann figures, with the clubs and stuff?
If the former, the Slann are generally the Generals of Lizardmen armies, so they are unlikely to show up in a Mordheim skirmish game (which is why the Lizardmen warband is lead by a Skink Priest.)
If the latter, I'd just use them as 'counts as Skinks'.