is it possible to add a pump to increase waterpressure & flow? similar to
a shower pump for the whole house?

Exactly like a shower pump for the whole house, in fact. You may wish to
distinguish between outlets, however. The pump is noisy, which isn't a
problem with a shower or bath, but might wake people up if the turning on a
basin tap or the washing machine rinsing at night turns on the pump.

Christian.