The repeat orders checkbox in your task force orders screen is your friend here. Your cargo ships will likely not be bringing full loads but they should get it done. Let's assume you have 3 mining operations you'd like to gather from, so you create a cargo group and you'd set up orders like so.
Load all minerals site 1
Load all minerals site 2
Load all minerals site 3
Unload all minerals Earth
Refuel from Earth
With the repeat orders checkbox checked then your group will immediately start over with the first order again. Obviously site 1,2 etc get replaced with your actual mining colonies, Luna, Titan, Asteroid 47, or whatever.
The only issue here is if your ships run out of fuel, hence the Refuel order, or if they get too full on their route, in which case they will start canceling orders, if that happens either add more cargo ships to the group, or reduce the route.
Hope that helps.
