can u add a new fuel pump while keeping the old one in place, i mean can i put an inline fuel pump in as well as leaving the old one in?

Depends on what your reasons are for having an external pump... If the reason is due to the original pump working correctly but not being able to supply enough fuel pressure for your application, then yes you can run an external pump to raise the pressure. Depending on your application you can also try a fuel pressure regulator to raise the pressure up. If your trying to place an external pump due to the original failing, then it is only a matter of time before it fails completely and it won't be able to supply fuel to the secondary pump at all. If the original is dying, you could just completely remove it and use an external pump only, but whilst your in the fuel tank you might as well replace it with another internal one. Have a look at some surge tank setups, most run an external pump or two with the only difference being that there is an extra tank there that you won't be wanting. IE - My skyline has an aftermarket (larger) bosch intank pump which is not adequate for my power, I then have another Bosch external pump mounted outside the tank (with the surge tank) to increase the pressure to the fuel rail, and on top of that I then have a fuel reg to control the flow to what I want it to be once it's at the rail.