Transformation of soil moisture units
The other answers are correct that you need to know the soil depth to convert from mass units (kg m-2) to volumetric units (m3 m-3).  However, when comparing these variables you should also be aware that satellite soil moisture is typically measured only over the top few centimetres of soil. Total column soil moisture from a climate model will typically be the sum over several metres, so the dynamics of these two variables are likely to be quite different (contrast the red and light blue lines in the upper plot).

It's more common to compare satellite data with top-layer soil moisture and even then to normalize each dataset in some way, e.g, 

$$NWI = \frac{SM - SM_{min}}{SM_{max} - SM_{min}}$$

or as normalised anomalies,

$$NWI = (SM - \overline{SM}) / \sigma_{SM}$$

I've used the latter to normalize the lower plot.  Normalization gets around the units problem, but doesn't get around the fact that you're comparing physically different quantities.