I would be curious why the soil is wet under the foundation in the Winter. I can understand the summer times.
Is the add on area where the add on's attach to the original home shored up by post or beams or piers? Sounds like without seeing that the additions are pulling down on the original home and causing the cracks.
I almost sense that you live in Texas or the southwest area.
In the summer, you need to surround the home, 16 inches from the foundation, black soaker hoses. During the dry and hot months, you need to water one half the foundation one day and the other half the next day. Do the watering at night so that evap is to a min. Do this once every 10 days or so. This will help you with settling.
Remember the home will heave up in the Winter and fall in the summer. You need to minimize this during the times of heat and lack of rain.
If this does not work, then its time to think about jacks for the additions. They run about $500 per jack and one is needed every 10 feet around the area.