Ok finally found the solution myself. Here it is :
SELECT ROUND(CONVERT(numeric(19,6), $[OITB.U_TYFreightAllowance]) + CONVERT(numeric(19,6), $[OITB.U_TYDouane]) + CONVERT(numeric(19,6), $[OITB.U_TyTransport]) + CONVERT(numeric(19,6), $[OITB.U_TYTauxChange]) + CONVERT(numeric(19,6), $[OITB.U_MPPrix1CAD]), 2) AS Total
Thank you to this thread : Helpful thread