Can I substitute an in-line fuelpump for the.

There used to be aftermarket inline pumps available at hardware stores but I haven't seen them in years. You have to know which engine size you have, the type of fuel injection system, the needed fuel system pressure, and whether the pressure regulator is on the fuel rail on the engine or built into the pump assembly. Assuming you can find a pump that develops the right pressure, it needs to change up and down in most cases to adjust for changes in intake manifold vacuum. If the pump can't respond and build pressure fast enough during acceleration, you will have a stumble or hesitation.

You would also have to wire the pump to only turn on at the times the manufacturer designed it to. If it runs whenever the ignition switch is on you will have a major fire hazard if a fuel line gets ruptured in a crash. You also have to consider that wherever you cut the line to install the pump, you'll have to prepare the ends so the hose doesn't push off from the pressure. By the time you're done with all the work, you could have installed the pump in the tank, everything will fit the first time, and it will work right. Lawyers and insurance investigators love to find modifications to safety systems, especially when they're trying to shift the blame from the guy who caused a crash. My vote is to stick with what GM designed.