I've just noticed that your proposed other adapter doesn't specify that the output is DC. That could be critical, as if the output is AC it definitely won't be suitable.
The requirements of a power supply are as follows:
a) It must regulate and maintain the voltage within the expected input range and polarity of the powered device;
b) It must provided the current asked for by the powered device while maintaining the expected voltage;
c) It must provide current of the quality required by the powered device, smoothed and ripple free to the necessary extent.
It is obviously possible that any substituted power adapter may fail on any of these points.
If you have a multimeter and a small 12 V bulb (e.g. a 6 W automobile bulb) you could do some tests.
To check for AC/DC, use a DC voltage range on the meter and verify that it reads approximately +12 V when measured the right way round, and -12 V when measured the wrong way round. (By right way round, I mean the connection to the power plug should be made the same way round as on the Maha adapter.)
To check for voltage regulation, measure the voltage open circuit and also when powering the small bulb. The voltage in both cases should be in the 12-14 V range.
To check for smoothing is more difficult, but that may not be too critical for most applications. One way is to divide the voltage down to about 1 V and connect it to the line input of an audio device to see whether you get a loud hum or not. But don't do that unless you know what you are doing.
As to your specific question about topping off, it will work fine as long as the substitute power adapter is suitable overall.
Basically, as long as the charger doesn't ask for more than 500 mA and the adapter can supply 500 mA at the right voltage and power quality it will be OK. The charger is unlikely to ask for more than 500 mA unless you use the fast charge mode.
If this all sounds complicated, it is. If you really want to be comfortable, go out and buy a regulated universal DC power adapter designed for travel and use that.
I don't want to be responsible for you damaging your charger, so if you are scared of something going wrong make sure you can afford to replace the charger before you experiment.
