You cannot fit a pump into a pressurised system for two reasons.
Firstly pumps are only rated between 1 or 2 bar where mains can be as high as 10 bar. The pump seals would fail.
Secondly, Pumps pump not suck. If you have low water pressure, a pump will not overcome this. It could cause all sorts of problems like air entering when a sink tap is turned on or ground water being sucked into a leaking water main.
Combi's should give an excellent shower. If the water pressure is low, you need to find out why and cure that problem.
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