If you think about it, liquid hydrogen isn't very compatible with spaces that normally open into the living spaces of the ship, so you wouldn't/couldn't just pump the fuel into the cargo space (which normally connects via internal corridors and doors into human-livable spaces, and is pressurized and heated).
In the original GDW Traveller (under one of the many supplements, as I recall), it was possible to install additional fuel tankage in cargo space -- but this tankage wasn't just a matter of loading fuel into the cargo space, it was actual tanks that were installed through the loading doors or hatches and plumbed into the ship's fuel system. This would allow exactly what you suggest, making multiple jumps with internal fuel. A shipyard or repair facility could install or remove the tanks, at a cost in time and money of course.
There was also an option (from High Guard, I think) to use "drop tanks" -- jettisonable external tanks that could be used to charge the jump system, then dropped just before jumping to allow arriving with full tanks (less the tiny quantity of fuel used for power generation while in jump) that would permit a second immediate jump.
Alternatively, there was an option for the original "Scout" class ships (J2 but with no cargo capability) to convert a portion of tank space to human-conditioned cargo capacity (if the J2 drive wasn't downgraded to J1, the ship could make a J2 by using drop tanks). Collapsible tanks were more expensive, but need not be jettisoned, merely retracted once empty and before initiating jump.
Range extension could also be managed by fuel scooping -- jumping to a system with either a gas giant (virtually all systems have one or more) where the ship would dive into the atmosphere of impure hydrogen and compress the "free" fuel, or a supply of water (ice from comets or oceans) to refill the tanks (though if the ship wasn't equipped with a military grade fuel processor, the low fuel quality could induce a misjump).
Whether any of these rules made it into the current Mongoose version of the game I don't know, but it should be easy to house-rule any or all of these options to allow greater exploration or trading range with an affordable ship.