Has been asked for countless times.
Is never going to happen.
Hopefully, as it would utterly unbalance things.
Simple example:
A cmdr now equips a 5D Hull Reinforcement Package in a size 5 slot.
Mass: 16t, Hull Reinforcement: 390, Resistances: 2.5%
He engineers it with a G5 Heavy Duty mod.
Maxed out roll without secondaries gives him
Mass: 22.4t, Hull Reinforcement: 624, Resistances: 16.5%
Now, with your suggestion implemented, the cmdr fits 5x 1D Hull Reinforcement Packages in the same size 5 slot.
Mass: 1t, Hull Reinforcement: 110, Resistances: 0.5% per package
All 5 together:
Mass: 5t, Hull Reinforcement: 550, Resistances: 2.47%
He engineers all of them with a G5 Heavy Duty mod.
Maxed out rolls without secondaries give him
Mass: 1.4t, Hull Reinforcement: 176, Resistances: 14.5% per package
All 5 together:
Mass: 7t, Hull Reinforcement: 880, Resistances: 54.3%
In short, a single size 5 slot would give you a whopping 54.3% resistances instead of 16.5% while even reducing the weight from 22.4t to only 7t.