This is something that's been requested for a while I think. Hopefully its availability in SP means the framework should already be there to move it into MP. I really hope it isn't members only though. IMO, it should be unlocked at the same level as Custom Scenario, as it's pretty much a part of what Custom Scenario already should have been.https://warlight.uservoice.com/forums/77051-warlight-features/suggestions/5201249-the-ability-to-assign-cards-to-slots
There's already a uservoice , by the way. Check it out if you like this idea.