I have just realised that I have only added half of the required amount of spray malt to my beer kit on day 7 of fermentation. Can I just stir in another bag of spray malt after racking it off the yeast cake and continue to brew it in the beer bucket with airlock. Would I still add sugar again before putting it into the pressure barrel.
I would boil the spray-malt in some water. This will both dissolve and sanitise the malt. As soon as it is at roughly the same temperature as the ferment, add it in and stir well, but minimising splashing as much as possible, so as not to re-introduce too much extra oxygen. The existing yeast should have no trouble consuming the new sugars. There should already be roughly enough yeast, so I would not expect another large growth phase (which is why we minimised the splashing/O2).
Consider your brew to have a delayed fermentation timeline, maybe add another week to the ferment-time-estimate. Because you are adding water along with the spray malt, the original gravity should be slightly less than what it would have been with the malt all added at the beginning. This change will be reflected in your final gravity too.
But overall it shouldn't cause a big problem to your brew.