Pumps have a maximum flow rate, if the flow rate of your filter is equal to or more than the pump, it won't do anything. Additionally, the pump won't change the amount of filtration.
If the pump has a flow rater greater than the filter and you run it in line:
If it's before the filter, when the water hits the filter it's flow rate will decrease and it's flow rate after the filter will be limited by the filter's flow rate.
If it's after the filter, the pump will push water into the aquarium faster, but it will have no effect on the flow rate through the filter.
Pumps push, they don't suck.