Can 7/8 stub kit be step be fitted to a 3/4 suction line

Yes IF the size called for to operate on the unit is 3/4 OD. This way you are not restricting the line size.

I have a 3.5 ton heat pump suction line 7/8" I have evapor coil suction line 3/4 will it work the coil is 3 ton both units is 13 seer