How to add solar panel?

[raed2014]

I have a cheap small battery backed solar panel water feature pump, the solar panel size is too small to allow enough current when the weather is overcast, is it possible to add extra solar panel(s) to gather enough current on overcast days?
And if it is doable, what would I need and how do I go about putting it together?

Pump: 3.7V, 0.9W

Power of Solar Battery: 6V/3W

Size of solar panel: 150*180*18mm

Thanks