Will a 42000mah portable power bank charge an electric bike battery?
The powerbank can (apparently) output enough volts and watts to power your charger, so yeah, sure, it could charge your ebike battery. The real question is: would it be a good idea to do that?

Most likely the ebike battery manufacturer has not foreseen the use of a charger on a moving bike, so I would not bet on cell balancing and other "details" to work properly. But that's a minor detail, if the battery bursts into flame, the brakes will still work.


  The electric bike I have has a 48v , 15.4ah battery.


That's about 740 Wh, which is rather huge for an ebike.

Note: 

current * voltage = power

power * time = energy

current * voltage * time = energy

Ah = current * time (ie, amperes * hours)

Ah * volts = current * time * volts = energy 

(if current is in A, time in hours, and voltage in volts, then energy is in watts*hours)

The powerbank weighs about 1 kilo, so let's say it contains about 800 grams of batteries (I'm being generous) if the cells are good quality, at 200 Wh/kg, then it will hold about 160 Wh of energy. This is small compared to the main ebike battery, so even if the thing works, it won't make a big difference.

Note:

Ah is the unit for the capacity of a battery, but it doesn't say how much energy it contains. Most likely the monkey who wrote the marketing blurb for your powerbank went like so:

"It contains 14x 3.0Ah 3.6V LiIon cells, so its capacity is 42Ah, so 42000 mAh! More zeros is more good! Yay!!!!!"

Note 42 Ah x 3.6V = 150 Wh which is pretty close to my 160 Wh wet-finger-in-the-wind estimation by weight.

This is slightly different from what we engineers call a "specification". "150 Wh" is a specification, it tells you how much energy is stored in the box. "42Ah" does not tell you how much energy is stored, only the total battery capacity. If you want to know the energy you have to multiply by the battery voltage, which is not mentioned. The output voltage is mentioned, but it has no relation to the voltage on the internal battery, since there will be a converter.

TL/DR: 

1) This will not extend the range of your ebike by enough km to be worth the headache of making it work.

2) Purchasing a spare battery compatible with your ebike (even one of lower capacity) is a much better idea, because it will work. Also you'r only paying for battery, not for voltage conversion electronics.