can i put barrier tokens over smoke cloud markers?

No. Barrier tokens has to be placed on clear terrain. Smoke Cloud markers are by their very nature Hindering terrain.
However, Smoke Cloud tokens can be placed on hindering terrain, such as other Smoke Cloud tokens...

Nope. From DRPAC: "BARRIER (OPTIONAL): Give this character a power action; place up to four barrier terrain markers (which are blocking terrain) on the battlefield in any square of clear terrain that is not occupied by a character and is within this character’s range." Since Smoke Cloud markers are considered to be hindering terrain, you can't.

EDIT: too slow .