In other programs, generating cumulus clouds is typically done with a fluids generator - which Carrara doesn't have. 

Cumulus themselves can be done with a spline base shape and a couple of surface replicators using spheres with custom texutres. It takes sometweaking to get the right look but is very do-able. What would be a little more difficult to pull of is the soft, misty look. For that, a clout object would be the best. integrating the two might take awhile.