You could use:
$$\theta''=\theta''\cdot\frac{1'}{60''}\cdot\frac{1^{\circ}}{60'}\cdot\frac{\pi\text{ rad}}{180^{\circ}}=\frac{\pi}{648000}\theta\text{ rad}$$
Do you see how I used the fact that there are 60 arc seconds per 1 arc minute, 60 arc minutes per degree and 180 degrees per $\pi$ radians to construct 3 fractions all equal to 1 which I then multiplied with the original angle $\theta$, given in arc seconds, to obtain a conversion factor of $$\frac{\pi}{648000}$$?