Yes, you can use cell load sharing. Just remember to set HOCLSACC for the 1800 to ON.
One other parameter you can use to shift traffic is OFFSET or KOFFSET (depending on whether you are using ericsson 1 or 3 locating algorithm). If you want to shift traffic from cell1( 900cell) to cell2 (1800 cell), you can increase the OFFSETN of cell1-cell2 neighbour relation.
However, I think it’s best to use HCS to share the traffic.Are your 900 and 1800 cells in the same layer? Are they in the same HCSBAND? Also, what is the LAYERTHR of the 900 cell?