Fitting experimental curve with two physical equations including a ODE equation

Hi, I obtained a material mechanics stress-strain curve from experiment and hope to fit it with physical functions.

if y=stress; x=strain;

the functions are y=k0+sqrt(t); dtdx=k1*sqrt(t)-k2*t;

$$ 't' does not mean 'time' here, 't' represents dislocation density in the equations. k0, k1,k2 are parameters need to be fitted.

The experimental data is not presenting here cos the data is too many and the shape of the curve is a typical stress-strain curve.

1. Firstly I tried to use Curve Fitting Tool, but the problem is the cftool allow you to input only ONE customised function, as far as I see. Is that true?
2. Secondly, how to write scripts to fit curve with two or even more equations? especially one of them is ODE.

I appreciate any useful suggestions.